Methods for Nanowire Alignment and Deposition

ABSTRACT

The present invention provides methods and systems for nanowire alignment and deposition. Energizing (e.g., an alternating current electric field) is used to align and associate nanowires with electrodes. By modulating the energizing, the nanowires are coupled to the electrodes such that they remain in place during subsequent wash and drying steps. The invention also provides methods for transferring nanowires from one substrate to another in order to prepare various device substrates. The present invention also provides methods for monitoring and controlling the number of nanowires deposited at a particular electrode pair, as well as methods for manipulating nanowires in solution.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/979,949 filed Nov. 9, 2007 (now allowed) and claims benefit under35 U.S.C. 121. The disclosure of U.S. patent application Ser. No.11/979,949 is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to nanowires, and more particularly, tonanowire deposition and alignment.

2. Background of the Invention

Nanostructures, and in particular, nanowires have the potential tofacilitate a whole new generation of electronic devices. A majorimpediment to the emergence of this new generation of electronic devicesbased on nanostructures is the ability to effectively align and depositnanowires on various substrates. Electric fields allow for alignment ofnanowires suspended in suspension, but current techniques pose stringentconstraints on the scalability to large area substrates.

What are needed are systems and methods for achieving a high qualitynanowire deposition suitable for manufacturing large arrays ofnanostructure-enabled devices.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides methods forpositioning one or more nanowires. In suitable embodiments, thesemethods comprise providing one or more nanowires proximate to anelectrode pair and then energizing the electrode pair, whereby thenanowires become associated with the electrode pair. The energizing isthen modulating between the electrode pair, whereby the nanowires becomecoupled onto the electrode pair. In exemplary embodiments, the nanowiresare provided in a suspension. In suitable embodiments, the energizingcomprises generating an AC electric field between the electrode pair.The AC electric field can be generated using any method known in theart, for example, by supplying a signal to the electrode pair using adirect electrical connection or supplying an electromagnetic wave to theelectrode pair. In exemplary embodiments, an AC electric field of about10 Hz to about 5 kHz and about 0.5 V to about 3 V, is generated.Modulation of the AC field suitably comprises adjusting the frequency ofthe AC electric field, adjusting the amplitude of the AC electric field,or both, and in suitable embodiments, comprises increasing the frequencyof the AC electric field to from about 1 kHz to about 500 kHz,increasing the amplitude of the AC electric field to from about 2 V toabout 20 V, and/or increasing the frequency of the AC electric field tofrom about 500 Hz to about 100 kHz, followed by increasing the amplitudeof the AC electric field to from about 1 V to about 4 V. In embodimentswhere the electric field is supplied using an electromagnetic wave, afrequency of about 1 GHz to about 5 GHz is suitably used. The electrodesof the electrode pair can be separated by a distance that is less than,or equal to, a long axis length of the nanowires, for example.

The methods of the present invention can also further comprise removingone or more uncoupled nanowires from the electrode pair, for example byflushing away uncoupled nanowires, and can also further comprise dryingthe one or more coupled nanowires. In additional embodiments, themethods comprise repeating the association and modulation phases of themethods. In embodiments, the methods of the present invention furthercomprise transferring the one or more coupled nanowires onto a substrateand/or removing the electrode pair.

Nanowires that can be positioned according to the methods of the presentinvention include nanowires that comprise a semiconductor core (e.g.,Si) and one or more shell layers disposed about the core, for examplemetal shell layers such as TaAlN or WN.

In an additional embodiment, the present invention provides methods forpositioning one or more nanowires on a substrate. In suitableembodiments, the methods comprise providing one or more nanowires in asuspension proximate to an electrode pair on a transfer substrate andthen energizing the electrode pair, whereby the nanowires becomeassociated with the electrode pair. The energizing is then modulatedbetween the electrode pair, whereby the nanowires become coupled ontothe electrode pair. One or more uncoupled nanowires are then removed andcoupled nanowires are then transferred from the transfer substrate ontothe substrate. In suitable embodiments, the energizing comprisesgenerating an alternating current (AC) electric field between theelectrode pair.

The present invention also provides methods for positioning one or morenanowires. In such methods, one or more nanowires are provided proximateto an electrode pair. The electrode pair is then energized, whereby thenanowires become associated with the electrode pair, wherein one or moremetallic elements are positioned between electrodes of the electrodepairs, such that inter-nanowire distances between adjacent associatednanowires vary by less than about 50% of a standard deviation. Infurther embodiments, the methods further comprise modulating theenergizing between the electrode pairs, whereby the nanowires becomecoupled onto the electrode pair, and wherein one or more metallicelements are positioned between electrodes of the electrode pairs, suchthat inter-nanowire distances between adjacent coupled nanowires vary byless than about 50% of a standard deviation.

The present invention also provides substrates comprising at least afirst pair of electrodes and at least four nanowires coupled between thefirst pair of electrodes, wherein the inter-nanowire distances betweenadjacent coupled nanowires varies by less than about 50% of a standarddeviation. In suitable embodiments, the substrates further comprisethree or more metallic elements positioned between the electrodes of thefirst electrode pair.

The present invention also provides one or more electrode pairscomprising one or more nanowires positioned according to the methods ofthe present invention, as well as substrates comprising one or morenanowires positioned according to the methods of the present invention.

In a still further embodiment, the present invention provides methodsfor controlling the number of nanowires positioned on an electrode pair.In suitable embodiments, the methods comprise positioning one or morenanowires according to the methods of the present invention and thenapplying a signal to the electrode pair. The signal is monitored at theelectrode pair and then the positioning stopped when the signal attainsa pre-set value (for example, by reducing the electric field between theelectrode pair, thereby stopping the positioning of nanowires on theelectrode pair). Exemplary signals that can be monitored include, butare not limited to, impedance, voltage, capacitance, current, etc.

The present invention also provides substrates comprising an electrodepair, wherein a pre-determined number of nanowires have been positionedon the electrode pair, and wherein the number of nanowires has beencontrolled according to the methods of the present invention. Forexample, the present invention provides substrates comprising at leastfour electrode pairs and at least four nanowires positioned on eachelectrode pair, wherein each of the electrode pairs comprisesubstantially the same number of nanowires. In suitable embodiments, thenumber of nanowires positioned on each of the electrode pairs deviatesby less than 30%, less than 20% or less than 10%.

The present invention also provides apparatuses and systems forpositioning nanowires on a substrate. In suitable embodiments, theapparatuses and systems comprise a suspension comprising a plurality ofnanowires and a substrate comprising one or more electrode pairs. Theapparatuses and systems also suitably comprise a source for generatingan alternating current (AC) electric field between the electrode pairs,for example a signal generator that is also used to modulate the ACelectric field. In additional embodiments, the systems and apparatusesfurther comprise means for flowing the nanowire suspension over at leastone of the electrode pairs (e.g., a fluid flow control system, suitablyadapted to be coupled to an underside of the substrate), an opticalimaging system for visualizing the nanowires, and one or more fieldelectrodes for manipulating the nanowires on the substrate. The systemsand apparatuses can also further comprise a signal monitoring device fordetermining the signal at the one or more electrode pairs and means forstopping the AC electric field when the signal attains a pre-set value.

The present invention also provides methods for depositing one or morenanowires on a substrate by heating the nanowires, such that they becomedeposited on the substrate. In exemplary embodiments, the nanowires areheated to about 200° C. in the presence of H₂ gas (forming gas) in orderto deposit them on the substrate.

In a still further embodiment, systems for manipulating nanowires areprovided. Such systems comprise one or more electrode sets, eachelectrode set comprising a first electrode having a first polarity and asecond electrode having a second polarity. The systems also comprise asignal generator for generating an alternating current (AC) electricfield between the first and second electrodes. Methods for manipulatingnanowires utilizing such systems are also provided. For example, anelectrode set is energized, and then suitably de-energized, while anadjacent electrode set is energized. This produces a delectophoreticforce that manipulates the nanowires in the direction of the energizing.In additional embodiments, a removal electrode can be utilized, whichwhen energized, manipulates nanowires in the direction of the removalelectrode, thereby allowing the nanowires to be removed from electrodepairs used in association/coupling applications. Both DC and AC electricfields can be used to manipulate the nanowires.

The present invention also provides methods for separating one or moreconductive nanowires from a mixture of conductive and semiconductivenanowires. While both conductive and semiconductive nanowires can beassociated with electrode pairs, conductive nanowires become coupled atamplitudes that are generally lower than those required for coupling ofsemiconductive nanowires, thus allowing for selective removal ofconductive nanowires from a solution.

In further embodiments, the systems for positioning nanowires on asubstrate of the present invention can also further comprise one or morenanowire-adhering regions on the substrate. Such systems can be used invarious methods for positioning one or more nanowires, for example,where uncoupled nanowires are removed by attaching to thenanowire-adhering regions.

Further embodiments, features, and advantages of the invention, as wellas the structure and operation of the various embodiments of theinvention are described in detail below with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described with reference to the accompanying drawings.In the drawings, like reference numbers indicate identical orfunctionally similar elements. The drawing in which an element firstappears is indicated by the left-most digit in the correspondingreference number.

FIG. 1A is a diagram of a single crystal semiconductor nanowire.

FIG. 1B is a diagram of a nanowire doped according to a core-shell (CS)structure.

FIG. 1C is a diagram of a nanowire doped according to a core-shell-shell(CSS) structure.

FIG. 2 shows an apparatus for aligning and depositing nanowires inaccordance with one embodiment of the present invention.

FIG. 3 is a schematic showing the effect of an electric field on thecharge separation in a nanowire.

FIG. 4 is a schematic showing the rotational torque exerted on ananowire.

FIG. 5 is a schematic showing the alignment and association of ananowire with an electrode pair in accordance with one embodiment of thepresent invention.

FIG. 6 is a plot showing the effect of nanowire length on thepolarization function of nanowires as a function of frequency.

FIG. 7 is a plot showing the effect of nanowire composition on thepolarization function of nanowires as a function of frequency.

FIG. 8 a is a micrograph showing the alignment and association ofnanowires with electrode pairs.

FIG. 8 b is a micrograph showing the coupling of nanowires ontoelectrode pairs.

FIG. 8 c is a micrograph showing the removal of uncoupled nanowires fromelectrode pairs.

FIG. 8 d is a micrograph showing nanowires coupled to electrode pairsfollowing drying.

FIG. 9 a is a micrograph showing 11 electrode pairs comprising aligned,coupled nanowires.

FIG. 9 b is a micrograph showing 10 electrode pairs comprising a highdensity of aligned, coupled nanowires.

FIG. 10 a is a micrograph showing associated nanowires prior to themodulation alignment phase.

FIG. 10 b is a micrograph showing the same nanowires as in FIG. 10 a,following the modulation alignment phase.

FIG. 10 c is a micrograph showing the same nanowires as in FIG. 10 b,following the coupling phase.

FIG. 11 is a schematic showing the coupling of a nanowire onto anelectrode pair in accordance with one embodiment of the presentinvention.

FIG. 12 is a micrograph showing nanowires coupled in both the x and yplanes.

FIG. 13 is a micrograph showing nanowires aligned with the aid ofmetallic elements positioned between electrodes, in accordance with oneembodiment of the present invention.

FIG. 14 a represents a flowchart showing a method of nanowire alignmentand deposition in accordance with one embodiment of the presentinvention.

FIG. 14 b represents a nanowire alignment and deposition sequence inaccordance with one embodiment of the present invention.

FIG. 14 c represents an additional nanowire alignment and depositionsequence in accordance with one embodiment of the present invention.

FIG. 15 is a schematic of an apparatus and method for nanowire transferin accordance with one embodiment of the present invention.

FIG. 16 a is a schematic showing an apparatus for monitoring thedeposition of nanowires onto a device under test in accordance with oneembodiment of the present invention.

FIG. 16 b is a schematic showing an additional apparatus for monitoringthe deposition of nanowires onto a device under test in accordance withone embodiment of the present invention.

FIG. 17A-17B represent e-field aligned nanowires prior to (A); andfollowing (B) heat deposition in accordance with one embodiment of thepresent invention.

FIG. 18 represents a schematic of a system for manipulating nanowires ina solution in accordance with one embodiment of the present invention.

FIG. 19 shows a flowchart of a method of manipulating nanowires inaccordance with one embodiment of the present invention.

FIG. 20 shows a schematic of a system for removing nanowires inaccordance with one embodiment of the present invention.

FIGS. 21A-21C represent the effects of the application of DC and ACelectric fields on nanowires in accordance with one embodiment of thepresent invention.

FIGS. 22A-22F show micrographs depicting nanowire association as well asnanowire removal, in accordance with one embodiment of the presentinvention.

FIG. 23 shows a schematic of a system for positioning nanowires inaccordance with one embodiment of the present invention.

FIG. 24 shows a flowchart of a method for positioning nanowires inaccordance with one embodiment of the present invention.

FIGS. 25A-25C show micrographs depicting nanowire association as well asnanowire removal, in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

It should be appreciated that the particular implementations shown anddescribed herein are examples of the invention and are not intended tootherwise limit the scope of the present invention in any way. Indeed,for the sake of brevity, conventional electronics, manufacturing,semiconductor devices, and nanowire (NW), nanorod, nanotube, andnanoribbon technologies and other functional aspects of the systems (andcomponents of the individual operating components of the systems) maynot be described in detail herein. Furthermore, for purposes of brevity,the invention is frequently described herein as pertaining to nanowires.

It should be appreciated that although nanowires are frequently referredto, the techniques described herein are also applicable to othernanostructures, such as nanorods, nanotubes, nanotetrapods, nanoribbonsand/or combinations thereof. It should further be appreciated that themanufacturing techniques described herein could be used to create anysemiconductor device type, and other electronic component types.Further, the techniques would be suitable for application in electricalsystems, optical systems, consumer electronics, industrial electronics,wireless systems, space applications, or any other application.

As used herein, an “aspect ratio” is the length of a first axis of ananostructure divided by the average of the lengths of the second andthird axes of the nanostructure, where the second and third axes are thetwo axes whose lengths are most nearly equal to each other. For example,the aspect ratio for a perfect rod would be the length of its long axisdivided by the diameter of a cross-section perpendicular to (normal to)the long axis.

The term “heterostructure” when used with reference to nanostructuresrefers to nanostructures characterized by at least two different and/ordistinguishable material types. Typically, one region of thenanostructure comprises a first material type, while a second region ofthe nanostructure comprises a second material type. In certainembodiments, the nanostructure comprises a core of a first material andat least one shell of a second (or third, fourth, fifth, etc.) material,where the different material types are distributed radially about thelong axis of a nanowire, a long axis of an arm of a branchednanocrystal, or the center of a nanocrystal, for example. A shell neednot completely cover the adjacent materials to be considered a shell orfor the nanostructure to be considered a heterostructure. For example, ananocrystal characterized by a core of one material covered with smallislands of a second material is a heterostructure. In other embodiments,the different material types are distributed at different locationswithin the nanostructure. For example, material types can be distributedalong the major (long) axis of a nanowire or along a long axis of arm ofa branched nanocrystal. Different regions within a heterostructure cancomprise entirely different materials, or the different regions cancomprise a base material.

As used herein, a “nanostructure” is a structure having at least oneregion or characteristic dimension with a dimension of less than about500 nm, e.g., less than about 200 nm, less than about 100 nm, less thanabout 50 nm, or even less than about 20 nm. Typically, the region orcharacteristic dimension will be along the smallest axis of thestructure. Examples of such structures include nanowires, nanorods,nanotubes, branched nanocrystals, nanotetrapods, tripods, bipods,nanocrystals, nanodots, quantum dots, nanoparticles, branched tetrapods(e.g., inorganic dendrimers), and the like. Nanostructures can besubstantially homogeneous in material properties, or in certainembodiments can be heterogeneous (e.g., heterostructures).Nanostructures can be, for example, substantially crystalline,substantially monocrystalline, polycrystalline, amorphous, or acombination thereof. In one aspect, each of the three dimensions of thenanostructure has a dimension of less than about 500 nm, for example,less than about 200 nm, less than about 100 nm, less than about 50 nm,or even less than about 20 nm.

As used herein, the term “nanowire” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) that includes at least one cross sectional dimension that isless than 500 nm, and preferably, equal to or less than less than about100 nm, and has an aspect ratio (length:width) of greater than 10,preferably greater than 50, and more preferably, greater than 100.Exemplary nanowires for use in the practice of the methods and systemsof the present invention are on the order of 10's of microns long (e.g.,about 10, 20, 30, 40, 50 microns, etc.) and about 100 nm in diameter.

The nanowires of this invention can be substantially homogeneous inmaterial properties, or in certain embodiments can be heterogeneous(e.g., nanowire heterostructures). The nanowires can be fabricated fromessentially any convenient material or materials, and can be, e.g.,substantially crystalline, substantially monocrystalline,polycrystalline, or amorphous. Nanowires can have a variable diameter orcan have a substantially uniform diameter, that is, a diameter thatshows a variance less than about 20% (e.g., less than about 10%, lessthan about 5%, or less than about 1%) over the region of greatestvariability and over a linear dimension of at least 5 nm (e.g., at least10 nm, at least 20 nm, or at least 50 nm). Typically the diameter isevaluated away from the ends of the nanowire (e.g., over the central20%, 40%, 50%, or 80% of the nanowire). A nanowire can be straight orcan be e.g., curved or bent, over the entire length of its long axis ora portion thereof. In certain embodiments, a nanowire or a portionthereof can exhibit two- or three-dimensional quantum confinement.Nanowires according to this invention can expressly exclude carbonnanotubes, and, in certain embodiments, exclude “whiskers” or“nanowhiskers”, particularly whiskers having a diameter greater than 100nm, or greater than about 200 nm.

Examples of such nanowires include semiconductor nanowires as describedin Published International Patent Application Nos. WO 02/17362, WO02/48701, and WO 01/03208, carbon nanotubes, and other elongatedconductive or semiconductive structures of like dimensions, which areincorporated herein by reference.

As used herein, the term “nanorod” generally refers to any elongatedconductive or semiconductive material (or other material describedherein) similar to a nanowire, but having an aspect ratio (length:width)less than that of a nanowire. Note that two or more nanorods can becoupled together along their longitudinal axis so that the couplednanorods span all the way between electrodes. Alternatively, two or morenanorods can be substantially aligned along their longitudinal axis, butnot coupled together, such that a small gap exists between the ends ofthe two or more nanorods. In this case, electrons can flow from onenanorod to another by hopping from one nanorod to another to traversethe small gap. The two or more nanorods can be substantially aligned,such that they form a path by which electrons can travel betweenelectrodes.

A wide range of types of materials for nanowires, nanorods, nanotubesand nanoribbons can be used, including semiconductor material selectedfrom, e.g., Si, Ge, Sn, Se, Te, B, C (including diamond), P, B—C,B—P(BP₆), B—Si, Si—C, Si—Ge, Si—Sn and Ge—Sn, SiC, BN, BP, BAs, AlN,AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, ZnO, ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS,MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl,CuBr, CuI, AgF, AgCl, AgBr, AgI, BeSiN₂, CaCN₂, ZnGeP₂, CdSnAs₂,ZnSnSb₂, CuGeP₃, CuSi₂P₃, (Cu, Ag)(Al, Ga, In, Tl, Fe)(S, Se, Te)₂,Si₃N₄, Ge₃N₄, Al₂O₃, (Al, Ga, In)₂ (S, Se, Te)₃, Al₂CO, and anappropriate combination of two or more such semiconductors.

The nanowires can also be formed from other materials such as metalssuch as gold, nickel, palladium, iradium, cobalt, chromium, aluminum,titanium, tin and the like, metal alloys, polymers, conductive polymers,ceramics, and/or combinations thereof. Other now known or laterdeveloped conducting or semiconductor materials can be employed.

In certain aspects, the semiconductor may comprise a dopant from a groupconsisting of: a p-type dopant from Group III of the periodic table; ann-type dopant from Group V of the periodic table; a p-type dopantselected from a group consisting of: B, Al and In; an n-type dopantselected from a group consisting of: P, As and Sb; a p-type dopant fromGroup II of the periodic table; a p-type dopant selected from a groupconsisting of: Mg, Zn, Cd and Hg; a p-type dopant from Group IV of theperiodic table; a p-type dopant selected from a group consisting of: Cand Si.; or an n-type dopant selected from a group consisting of: Si,Ge, Sn, S, Se and Te. Other now known or later developed dopantmaterials can be employed.

Additionally, the nanowires or nanoribbons can include carbon nanotubes,or nanotubes formed of conductive or semiconductive organic polymermaterials, (e.g., pentacene, and transition metal oxides).

Hence, although the term “nanowire” is referred to throughout thedescription herein for illustrative purposes, it is intended that thedescription herein also encompass the use of nanotubes (e.g.,nanowire-like structures having a hollow tube formed axiallytherethrough). Nanotubes can be formed in combinations/thin films ofnanotubes as is described herein for nanowires, alone or in combinationwith nanowires, to provide the properties and advantages describedherein.

It should be understood that the spatial descriptions (e.g., “above”,“below”, “up”, “down”, “top”, “bottom,” etc.) made herein are forpurposes of illustration only, and that devices of the present inventioncan be spatially arranged in any orientation or manner.

FIG. 1A illustrates a single crystal semiconductor nanowire core(hereafter “nanowire”) 100. FIG. 1A shows a nanowire 100 that is auniformly doped single crystal nanowire. Such single crystal nanowirescan be doped into either p- or n-type semiconductors in a fairlycontrolled way. Doped nanowires such as nanowire 100 exhibit improvedelectronic properties. For instance, such nanowires can be doped to havecarrier mobility levels comparable to bulk single crystal materials.

FIG. 1B shows a nanowire 110 doped according to a core-shell structure.As shown in FIG. 1B, nanowire 110 has a doped surface layer 112, whichcan have varying thickness levels, including being only a molecularmonolayer on the surface of nanowire 110.

FIG. 1C shows a nanowire 114 doped according to a core-shell-shellstructure. As shown in FIG. 1C, nanowire 114 has a doped surface layer112, which can have varying thickness levels including being only amolecular monolayer on the surface of nanowire 114, as well as an outershell layer 116. Exemplary materials for use as outer shell layer 116include, but are not limited to, TaAlN and WN.

The valence band of the insulating shell can be lower than the valenceband of the core for p-type doped wires, or the conduction band of theshell can be higher than the core for n-type doped wires. Generally, thecore nanostructure can be made from any metallic or semiconductormaterial, and the shell can be made from the same or a differentmaterial. For example, the first core material can comprise a firstsemiconductor selected from the group consisting of: a Group II-VIsemiconductor, a Group III-V semiconductor, a Group IV semiconductor,and an alloy thereof. Similarly, the second material of the shell cancomprise a second semiconductor, the same as or different from the firstsemiconductor, e.g., selected from the group consisting of: a GroupII-VI semiconductor, a Group III-V semiconductor, a Group IVsemiconductor, and an alloy thereof. Example semiconductors include, butare not limited to, CdSe, CdTe, InP, InAs, CdS, ZnS, ZnSe, ZnTe, HgTe,GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, and PbTe. Asnoted above, metallic materials such as gold, chromium, tin, nickel,aluminum etc. and alloys thereof can be used as the core material, andthe metallic core can be overcoated with an appropriate shell materialsuch as silicon dioxide or other insulating materials

Nanostructures can be fabricated and their size can be controlled by anyof a number of convenient methods that can be adapted to differentmaterials. For example, synthesis of nanocrystals of various compositionis described in, e.g., Peng et al. (2000) “Shape Control of CdSeNanocrystals” Nature 404, 59-61; Puntes et al. (2001) “Colloidalnanocrystal shape and size control: The case of cobalt” Science 291,2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos et al. (Oct. 23, 2001)entitled “Process for forming shaped group III-V semiconductornanocrystals, and product formed using process”; U.S. Pat. No. 6,225,198to Alivisatos et al. (May 1, 2001) entitled “Process for forming shapedgroup II-VI semiconductor nanocrystals, and product formed usingprocess”; U.S. Pat. No. 5,505,928 to Alivisatos et al. (Apr. 9, 1996)entitled “Preparation of III-V semiconductor nanocrystals”; U.S. Pat.No. 5,751,018 to Alivisatos et al. (May 12, 1998) entitled“Semiconductor nanocrystals covalently bound to solid inorganic surfacesusing self-assembled monolayers”; U.S. Pat. No. 6,048,616 to Gallagheret al. (Apr. 11, 2000) entitled “Encapsulated quantum sized dopedsemiconductor particles and method of manufacturing same”; and U.S. Pat.No. 5,990,479 to Weiss et al. (Nov. 23, 1999) entitled “Organoluminescent semiconductor nanocrystal probes for biological applicationsand process for making and using such probes.”

Growth of nanowires having various aspect ratios, including nanowireswith controlled diameters, is described in, e.g., Gudiksen et al (2000)“Diameter-selective synthesis of semiconductor nanowires” J. Am. Chem.Soc. 122, 8801-8802; Cui et al. (2001) “Diameter-controlled synthesis ofsingle-crystal silicon nanowires” Appl. Phys. Lett. 78, 2214-2216;Gudiksen et al. (2001) “Synthetic control of the diameter and length ofsingle crystal semiconductor nanowires” J. Phys. Chem. B 105, 4062-4064;Morales et al. (1998) “A laser ablation method for the synthesis ofcrystalline semiconductor nanowires” Science 279, 208-211; Duan et al.(2000) “General synthesis of compound semiconductor nanowires” Adv.Mater. 12, 298-302; Cui et al. (2000) “Doping and electrical transportin silicon nanowires” J. Phys. Chem. B 104, 5213-5216; Peng et al.(2000) “Shape control of CdSe nanocrystals” Nature 404, 59-61; Puntes etal. (2001) “Colloidal nanocrystal shape and size control: The case ofcobalt” Science 291, 2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos etal. (Oct. 23, 2001) entitled “Process for forming shaped group III-Vsemiconductor nanocrystals, and product formed using process”; U.S. Pat.No. 6,225,198 to Alivisatos et al. (May 1, 2001) entitled “Process forforming shaped group II-VI semiconductor nanocrystals, and productformed using process”; U.S. Pat. No. 6,036,774 to Lieber et al. (Mar.14, 2000) entitled “Method of producing metal oxide nanorods”; U.S. Pat.No. 5,897,945 to Lieber et al. (Apr. 27, 1999) entitled “Metal oxidenanorods”; U.S. Pat. No. 5,997,832 to Lieber et al. (Dec. 7, 1999)“Preparation of carbide nanorods”; Urbau et al. (2002) “Synthesis ofsingle-crystalline perovskite nanowires composed of barium titanate andstrontium titanate” J. Am. Chem. Soc., 124, 1186; and Yun et al. (2002)“Ferroelectric Properties of Individual Barium Titanate NanowiresInvestigated by Scanned Probe Microscopy” Nanoletters 2, 447.

Growth of branched nanowires (e.g., nanotetrapods, tripods, bipods, andbranched tetrapods) is described in, e.g., Jun et al. (2001) “Controlledsynthesis of multi-armed CdS nanorod architectures using monosurfactantsystem” J. Am. Chem. Soc. 123, 5150-5151; and Manna et al. (2000)“Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, andTetrapod-Shaped CdSe Nanocrystals” J. Am. Chem. Soc. 122, 12700-12706.

Synthesis of nanoparticles is described in, e.g., U.S. Pat. No.5,690,807 to Clark Jr. et al. (Nov. 25, 1997) entitled “Method forproducing semiconductor particles”; U.S. Pat. No. 6,136,156 to El-Shall,et al. (Oct. 24, 2000) entitled “Nanoparticles of silicon oxide alloys”;U.S. Pat. No. 6,413,489 to Ying et al. (Jul. 2, 2002) entitled“Synthesis of nanometer-sized particles by reverse micelle mediatedtechniques”; and Liu et al. (2001) “Sol-Gel Synthesis of Free-StandingFerroelectric Lead Zirconate Titanate Nanoparticles” J. Am. Chem. Soc.123, 4344. Synthesis of nanoparticles is also described in the abovecitations for growth of nanocrystals, nanowires, and branched nanowires,where the resulting nanostructures have an aspect ratio less than about1.5.

Synthesis of core-shell nanostructure heterostructures, namelynanocrystal and nanowire (e.g., nanorod) core-shell heterostructures,are described in, e.g., Peng et al. (1997) “Epitaxial growth of highlyluminescent CdSe/CdS core/shell nanocrystals with photostability andelectronic accessibility” J. Am. Chem. Soc. 119, 7019-7029; Dabbousi etal. (1997) “(CdSe)ZnS core-shell quantum dots: Synthesis andcharacterization of a size series of highly luminescent nanocrysallites”J. Phys. Chem. B 101, 9463-9475; Manna et al. (2002) “Epitaxial growthand photochemical annealing of graded CdS/ZnS shells on colloidal CdSenanorods” J. Am. Chem. Soc. 124, 7136-7145; and Cao et al. (2000)“Growth and properties of semiconductor core/shell nanocrystals withInAs cores” J. Am. Chem. Soc. 122, 9692-9702. Similar approaches can beapplied to growth of other core-shell nanostructures.

Growth of nanowire heterostructures in which the different materials aredistributed at different locations along the long axis of the nanowireis described in, e.g., Gudiksen et al. (2002) “Growth of nanowiresuperlattice structures for nanoscale photonics and electronics” Nature415, 617-620; Bjork et al. (2002) “One-dimensional steeplechase forelectrons realized” Nano Letters 2, 86-90; Wu et al. (2002)“Block-by-block growth of single-crystalline Si/SiGe superlatticenanowires” Nano Letters 2, 83-86; and U.S. patent application 60/370,095(Apr. 2, 2002) to Empedocles entitled “Nanowire heterostructures forencoding information.” Similar approaches can be applied to growth ofother heterostructures.

Electric Field Alignment and Deposition Overview

In one embodiment, the present invention provides methods for aligningand depositing nanowires from a suspended state onto a substratepatterned with electrodes in the presence of an electromagnetic field,as well as apparatuses and systems for performing such alignment anddeposition (see FIG. 2). Substrate and nanowire surface chemistryprovide a net electric charge on both the substrate and nanowires.Through appropriate choice of electrode pattern, the nanowires aresubjected to an electromagnetic field gradient that exerts a net forceon the nanowires. The force enables controlled manipulation of nanowires(e.g., in suspension) to specified locations on the substrate. Theelectrodes also generate an alternating current (AC) field thatpolarizes the nanowires, resulting in a net dipole moment. The AC fieldsubsequently exerts a torque on the dipole and enables an angularalignment parallel to the field direction. Appropriate choice ofelectrical parameters, e.g., frequency and amplitude, provide fornanowire alignment and capture, as well as a “pinning” or “association”of nanowires (NWs) over the electrodes. In the associated state, thenanowires are suitably aligned parallel to the electric field, but aresufficiently mobile along the electrode edges to accommodate achronological sequence of alignment and association events withoutgiving rise to nanowire clumping. The present invention also providesmethods for “locking” or “coupling” of nanowires onto the electrodes. Inthe coupled state, the nanowires remain aligned, as in the previouslyassociated state, but lose their lateral mobility along the electrodeedges. The present invention also provides for an appropriate choice offluid flow control to “flush” undesired/uncoupled/misaligned nanowiresfrom the electrodes. A drying process provides for removal of solventand enables an electrostatic “sticking” of the NWs on the surface.

Theoretical Discussion

The following section is included in order to provide some additionalbackground with regard to the theory behind electric field alignment. Itshould be understood that the present invention is not bound or limitedto the theory presented herein, and the ordinarily skilled artisan willreadily recognize that additional theories in addition to thosepresented herein are applicable to the present invention.

An external electric field {right arrow over (E)}_(EXT) causes a chargeseparation in a nanowire 208 as shown schematically in FIG. 3. Thischarge separation can be due to mobile charges (e.g., in a conductor) ordue to dipole moments (e.g., in a dielectric). The separated chargesproduce, within the nanowire, an induced electric field {right arrowover (E)}_(IND) that is in the direction opposite to the externalelectric field, and has a magnitude given by:

{right arrow over (E)} _(IND)=−ƒ(∈){right arrow over (E)} _(EXT);0≦ƒ(∈)≦1  (0.1)

The function ƒ(∈) depends on the permittivity c of the material and is ameasure of the polarizability of the nanowire. The upper and lowerlimits for the value of ƒ(∈) correspond to the limiting cases of amaterial compensating the external field completely and a materialhaving no compensating induced electric field, respectively. For amaterial containing either mobile charge (e.g., a metal) or dipolemoments (e.g., a dielectric) the induced electric field compensates theexternal electric field and results in a zero or reduced internalelectric field. Externally the charge separation or dipole momentorientation causes an induced dipole moment. The induced dipole moment P(as used throughout, bold, underlined characters represent vectors andcorrespond to those characters in the equations) of the nanowire isoriented along the external E-field direction and its magnitude isproportional to the nanowire polarizability κ(ω), nanowire volume V andelectric-field (E-field) strength {right arrow over (E)}:

{right arrow over (P)}=κ(ω)V·{right arrow over (E)}  (0.2)

A uniform E-field exerts a rotational torque T_(E) on the induced dipolemoment given by:

{right arrow over (T)} _(E) ={right arrow over (P)}×{right arrow over(E)} _(EXT)  (0.3)

The torque results in an energetically stable orientation for thenanowire 208 to be aligned parallel to the E-field direction, as shownin FIG. 4. A non-uniform E-field exerts a translational force F_(DEP)(dielectrophoresis) on the induced dipole moment given by:

{right arrow over (F)} _(DEP)=1/2Re[({right arrow over (P)}□{right arrowover (∇)}){right arrow over (E)}*]  (0.4)

Combining Eqs. (1.2) and (1.4), the dielectrophoretic force can beexpressed as:

{right arrow over (F)} _(DEP)=1/2π·r² L∈ _(m)·Re{κ(ω)}·{right arrow over(∇)}|{right arrow over (E)}| ²  (0.5)

The dielectrophoretic force on the nanowire 208 (see FIG. 5) depends onthe gradient of the quadratic E-field magnitude, E-field frequency ω,nanowire conductivity σ_(NW), nanowire permittivity ∈_(NW), solventpermittivity ∈_(m) and the nanowire shape (radius r and length L). Thereal part of the polarization function is given by:

$\begin{matrix}{{{Re}\left\{ \kappa \right\}} = \frac{{\omega^{2}\left( {{ɛ_{m}ɛ_{NW}} - ɛ_{m}^{2}} \right)} + \left( {{\sigma_{m}\sigma_{NW}} - \sigma_{m}^{2}} \right)}{{\omega^{2}ɛ_{m}^{2}} + \sigma_{m}^{2}}} & (0.6)\end{matrix}$

The frequency dependence of the polarization function yields twodistinct frequency ranges (low and high frequency limits), within whichthe dielectrophoretic force couples to two distinct properties of the NW(conductivity and permittivity). The cutoff frequency between the tworanges is given by the solvent or medium properties

$\begin{matrix}{f_{c} = \frac{\sigma_{m}}{2\pi \; ɛ_{m}}} & (0.7)\end{matrix}$

For f<<f_(c) the low frequency limit of the polarization function isgiven by:

$\begin{matrix}{{{Re}\left\{ \kappa \right\}} = \left\lbrack {\frac{\sigma_{NW}}{\sigma_{m}} - 1} \right\rbrack} & (0.8)\end{matrix}$

In this frequency range the dielectrophoretic force is proportional tothe conductivity of the NW normalized to the solvent conductivity. Forf>>f_(c) the high frequency limit of the polarization function is givenby:

$\begin{matrix}{{{Re}\left\{ \kappa \right\}} = \left\lbrack {\frac{ɛ_{NW}}{ɛ_{m}} - 1} \right\rbrack} & (0.9)\end{matrix}$

In this frequency range the dielectrophoretic force is proportional tothe permittivity of the NW normalized to the solvent permittivity. Thegeneral frequency dependence of the polarization function is plotted inFIG. 6 for two different NW lengths. In the low frequency range where NWconductivity is relevant, the polarization function does not depend onthe NW length. However, in the high frequency range the polarizationfunction scales ˜1/L² due to its dependence on the NW permittivity. InFIG. 7, the general frequency dependence of the polarization function isplotted for a given NW length but for two different NW conductivities(core-shell (CS) (Si) vs. core-shell-shell (CSS) (Si—TaAlN)). In the lowfrequency range the polarization function scales ˜σ_(NW) and in the highfrequency range the polarization is the same due to its dependence onthe NW permittivity.

Suitable Embodiments

In one embodiment, the present invention provides methods forpositioning one or more nanowires. In suitable embodiments, the methodscomprise providing one or more nanowires proximate to an electrode pair.Then, the electrode pair is energized, whereby the nanowires becomeassociated with the electrode pair. The energizing of the electrode pairis then modulated, whereby the nanowires become coupled onto theelectrode pair.

The term “positioning” as used throughout refers to the alignment andassociation, as well as the deposition or coupling, of nanowires onto asurface, for example, an electrode pair. Positioning includes nanowiresthat are both aligned and non-aligned. The term “aligned” as usedthroughout refers to nanowires that are substantially parallel ororiented in the same or substantially same direction of one another(i.e. the nanowires are aligned in the same direction, or within about45° of one another). Suitably, the nanowires of the present inventionare aligned such that they are all substantially parallel to one anotherand substantially perpendicular to each electrode of an electrode pair(though in additional embodiments, they can be aligned parallel to anelectrode). Positioning of nanowires onto an electrode pair suitablycomprises positioning the nanowires such that the nanowires span theelectrode pair, that is, the wires are in contact with both electrodesof an electrode pair (though wires can also be in contact with only oneelectrode), with the substantial remainder of their length between thetwo electrodes. In embodiments in which the nanowires are longer thanthe distance separating two electrodes of an electrode pair, thenanowires may extend beyond the electrodes.

Methods for providing nanowires for use in the methods and systems ofthe present invention are well known in the art. Suitably, the nanowiresare provided in a suspension, i.e., a suspension of nanowires comprisingone or more nanowires, suitably a plurality of nanowires, suspended in aliquid (i.e., a nanowire “ink”). Suitably, the liquid is an aqueousmedia, such as water or a solution of water, ions (including salts), andother components, for example surfactants. Additional examples ofliquids suitable for preparing nanowire suspensions include, but are notlimited, organic solvents, inorganic solvents, alcohols (e.g., isopropylalcohol) (IPA), etc.

As used herein the phrase “proximate to an electrode pair” as it relatesto providing the nanowires means that the nanowires are provided orpositioned such that they can be acted upon by an electric fieldgenerated at the electrode pair. Suitably, this is a distance from theelectrode pair such that they can be associated and coupled to theelectrodes. In more specific embodiments, the nanowires are providedsuch that they are at distance of less than about 100 μm from theelectrode pairs. For example, the nanowires are provided such that theyare less than about 100 μm, less than about 100 μm, less than about 50μm, less than about 10 μm, or less than about 1 μm, from the electodepair.

In suitable embodiments, the present invention provides a system orapparatus for nanowire alignment and deposition. An exemplary apparatus200 is shown in FIG. 2. Apparatus 200 suitably comprises a substrate 202having a surface onto which one or more electrodes are positioned (e.g.,patterned), suitably in an arrangement such that a positive electrode204 and a negative electrode 205 form an electrode pair 207. Substrate202 comprises any suitable material, for example a semiconductor waferor dielectric material. Examples of suitable substrate materialsinclude, but are not limited to, Si, SiO₂, GaAs, InP, and othersemiconductor materials described herein. Exemplary materials for use aspositive 204 and negative electrodes 205 include, but are not limitedto, Al, Mo (Moly electrodes), Cu, Fe, Au, Ag, Pt, Cr/Au etc. Electrodesfor use in the practice of the present invention can also furthercomprise an oxide coating or layer on their surface.

Nanowires of any material can be aligned and deposited according to themethods of the present invention. Suitably, the nanowires will comprisea semiconductor core and one or more shell layers disposed about thecore (i.e., the shell layers surround the core). Examples of suitablesemiconductor materials and shell materials include those describedthroughout. In suitable embodiments, the core comprises Si and at leastone of the shell layers, suitably the outermost shell layer (i.e., theshell layer that is in contact with the external environment) comprise ametal, such as TaAlN or WN. Additional examples of metal shell layersinclude those described throughout. Exemplary nanowires for use in thepractice of the present invention include core:shell (CS) nanowires(e.g., SiO₂), core:shell:shell (CSS) nanowires (e.g., SiO₂:metal), andcore:no oxide shell:metal shell nanowires (CNOS) (e.g., Si:metal). Infurther embodiments, additional shells (e.g., 3, 4, 5, 6, 7, etc.) canbe added to the nanowires, for example generating core:shell:shell:shell(CSSS) nanowires. In suitable embodiments, the outer shell is added soas to improve the zeta potential of the nanowire. In general, a negativezeta potential (usually high in magnitude) is desirable (though positivezeta potential can also be used), and thus, a final shell layer can beadded to generate such a zeta potential. For example, an oxide layer canbe added as the final shell layer of a nanowire. In suitableembodiments, the second to last outer shell can be a metal nitride, suchas tungsten nitride or tantalum nitride. This shell is then oxidized toform a final outer metal-oxide or metal-oxynitride shell on thenanowire. In further embodiments, a polysilicon or an oxide ofpolysilicon can be added as the final shell layer to aid in controllingthe zeta potential of the nanowires.

As shown in FIG. 2, in suitable embodiments, a plurality of electrodepairs 207 are patterned on substrate 202 such that they are adhered orfixed to the substrate. In other embodiments, electrode pairs 207 cansimply be layered on substrate 202, but not actually affixed to thesurface. Any suitable orientation or pattern of electrode pairs 207 canbe used. As shown in FIG. 2, apparatus 200 can also further compriseadditional pieces of material that form a flow channel 206 (e.g., twosides and a top (shown in a see-through view)). Flow channel 206provides a mechanism for controlling the amount and location of fluidflow when a suspension of nanowires is added to the apparatus. It shouldbe noted, however, that a flow channel is not required to perform thealignment and deposition methods of the present invention. Flow channel206 can be prepared using any suitable material, for example asemiconductor or dielectric material, and in suitable embodiments, flowchannel 206 is prepared from Polydimethylsiloxane (PDMS).

As noted above, suitably, one or more nanowires 208 are provided byproviding or introducing a suspension of nanowires to the apparatus 200(e.g., a nanowire “ink”). As represented in FIG. 2, suitably a nanowiresuspension is provided by flowing nanowires 208 over (arrows representan exemplary direction of flow, though other directions can also beused) the substrate surface 202 and the electrode pairs 207, and/or byutilizing a pressurized channel fill technique. The nanowire suspensionis maintained in the channel 206 during the providing phase. As thenanowires are provided, the flow of the suspension through channel 206and over electrode pairs 207 helps to align the nanowires, suitably inthe direction of the flow (e.g., the arrows shown in FIG. 2). Insuitable embodiments, channel 206 can be inverted so as to limit oreliminate gravity effects (i.e., the electrodes are on top of thechannel rather than the bottom, and thus nanowires do not settle on theelectrodes with gravity). Additional methods for providing nanowires toan electrode pair are well known in the art, and include, but are notlimited to, spray coaters, spray painting, meniscus coater, dip-coater,bar-coater, gravure coater, Mayer rod, doctor blade, extrusion,micro-gravure, web coaters, doctor blade coaters, in-line or ink jetprinters (see e.g, U.S. Pat. Nos. 6,936,761 and 6,358,643 thedisclosures of each of which are incorporated herein by reference).

As nanowires are provided to an electrode pair (e.g., via theintroduction of a suspension of nanowires to a channel 206), an electricfield is generated between the electrode pair by energizing theelectrode pair so as to associate the nanowires with the electrode pair.It should be noted that the electric field can be generated before,after, or during the period of nanowire producing/introduction. As usedherein, the terms “electric field” and “electromagnetic field” are usedinterchangeably and refer to the force exerted on charged objects in thevicinity of an electric charge. As used herein, “energizing theelectrode pair” or “energize” refer to any suitable mechanism or systemfor providing an electric current to the electrodes such that anelectric field is generated between electrodes of an electrode pair.

Suitably, energizing the electrode pair to generate the electric fieldis performed by generating an alternating current (AC) electric field(though a direct current (DC) electric field can also be used) eitherduring part or all of the alignment and deposition process. In suitableembodiments of the present invention, an electric field is generatedbetween a pair of electrodes (i.e., two electrodes) by applying anelectric current to the electrodes. For example, negative electrodes 205can be connected via direct electrical connection (i.e., wires or otherconnection) to a negative electrode terminal 210, to which the negativepole of an electric source is attached. Similarly, positive electrodes204 can be connected via direct electrical connection (i.e., wires orother connection) to a positive electrode terminal 212, to which thepositive pole of an electrode source is attached. When an electriccurrent is switched on, the negative and positive terminals thentransfer charge to the electrodes positioned on the substrate, therebygenerating an electric field between a pair of electrodes 207. Infurther embodiments, the electric field can be a pulsed electric field,for example a pulsed AC electric field.

The energizing of the electrode pair to create an electric field canalso be generated by supplying an electromagnetic wave to the electrodepairs 207. As is well known in the art, waveguides of various dimensionsand configurations (e.g., cylindrical, rectangular) can be suitably usedto direct and supply an electromagnetic wave (see e.g., Guru, B. S. etal., “Electromagnetic Field Theory Fundamentals,” Chapter 10, PWSPublishing Company, Boston, Mass. (1998)). Operation frequencies ofwaveguides for use in the practice of the present invention are readilydetermined by those of skill in the art, and suitably are in the rangeof about 100 MHz to 10 GHz, more suitably about 1 GHz-5 GHz, about 2-3GHz, about 2.5 GHz, or about 2.45 GHz.

As the nanowires encounter the AC electric field generated between thevarious pairs of electrodes, a field gradient results, as describedabove and represented in FIGS. 3 and 5. A net dipole moment is producedin the nanowires and the AC field exerts a torque on the dipole, suchthat the nanowires align parallel to the direction of the electric filed(FIG. 3). In one embodiment then, the present invention provides methodsfor aligning nanowires above one or more electrode pairs. It should benoted that the alignment and deposition methods of the present inventioncan be utilized on any nanowire composition, including, CS, CSS andCNOS.

In suitable embodiments, the electrodes of the electrode pair areseparated by a distance that is less than, or equal to, a long axislength of the nanowires. Nanowires of any length can be aligned andpositioned using the methods of the present invention. Suitably, thedistance between electrodes of an electrode pair are such that thenanowires extend just beyond the first edge of the electrode. That isthe distance between electrodes is about equal to, and suitably lessthan, the length of the nanowires being deposited. As shown in FIGS. 8a-8 d, 9 a-9 b and 10 a-10 c, nanowires suitably extend just beyond thefirst edge and into the middle of the electrode, with tens of nanometersto several microns overlapping the electrode material at the tip of thenanowire. Nanowires that are shorter than the distance between theelectrodes are able to couple to only one electrode in a pair (if theycouple at all), and suitably are removed during subsequent removingphases (since there is only one contact point between the electrode andthe nanowire). Similarly, nanowires that are substantially longer thanthe distance between the electrodes of an electrode pair hang over oneor more of the electrodes, and suitably are removed during subsequentremoving phases (larger exposed surface area). Thus, the methods of thepresent invention also provide a way to preferentially select nanowiresof a particular length from a suspension of a range of nanowire sizes,and align and deposit them onto an electrode pair.

It has also been determined that the methods of the present inventionpreferentially associate and couple nanowires that are “straight” ratherthan bent or crooked. Hence, the present invention also provides theadded benefit of depositing preferably straight nanowires, rather thanless preferred bent or crooked nanowires.

In additional embodiments, the electrodes used in the variousembodiments of the present invention can be different sizes, geometriesand orientations. For example, a first electrode (e.g., 204) cancomprise a greater nanowire contact surface area than a second electrode(e.g., 205) of an electrode pair 207. As used herein “greater nanowirecontact surface area” means that an electrode of an electrode pair has asurface area that is larger than the other electrode of the electrodepair, and hence, nanowires that are associated/coupled to the electrodepair contact a greater surface area of the first electrode. The use ofan electrode with a greater nanowire contact surface area allowsnanowires that are longer than the distance between the electrodes toalign substantially parallel to each other, as well as nanowires thatare more closely matched to the distance between electrodes of anelectrode pair. Thus, buy utilizing an electrode that has a greaternanowire contact surface area than the other electrode of the pair,nanowires that may normally misalign or cross, can now be aligned in asubstantially parallel fashion, thereby allowing association andcoupling of a population of nanowires that comprise various lengths.

In addition to aligning the nanowires parallel to the AC field, thefield gradient exerts a dielectrophoretic force on the nanowireattracting it toward the electrode pair. As represented schematically inFIG. 5, the gradient is highest at the electrode pair, and exerting anincreasing attraction toward the electrodes. An electric double-layer isproduced at the surface of each electrode of the pair, such thatoppositely charged ions are present at the electrode. In the presence ofthe electric field, the ions then migrate away from the electrode andinitially toward the nanowire hovering above. As ions approach theoppositely charged nanowire, the ions are repulsed by the like chargeand then directed back toward the electrode resulting in a circulatingpattern of ions. Liquid that is present (i.e., the nanowire suspension)is also circulated, generating an elecro-osmotic force that opposes thedielectrophoretic force attracting the nanowires to the electrodes. Asthe two forces reach an equilibrium (or relative equilibrium), thenanowires are held in place such that they become associated with theelectrode pair. As used herein the terms “associated” and “pinned” areused to indicate that the nanowires are in such a state that theelecro-osmotic force and the dielectrophoretic force are at equilibrium,such that there is no or little net movement of the nanowires away fromthe electrode pair (i.e., normal or substantially normal to thesubstrate and the electrode pair). This is also called the “associationphase” throughout.

In the associated, or pinned state, the nanowires are suitably alignedparallel to the electric field, but are sufficiently mobile along theelectrode edges (i.e. in a plane just above the surface of theelectrodes). For example, FIG. 8 a represents a set of four electrodepairs (204 and 205), each comprising a plurality of nanowires 208associated, or pinned, with the electrodes. The direction of theelectric field is shown in FIG. 8 a (the e-field can be in anydirection, though it will generally be in a direction that issubstantially perpendicular to the electrodes). As shown in FIG. 8 a,the majority of the nanowires are aligned in a direction parallel to thee-field (i.e. across or perpendicular to the electrodes). In embodimentswhen a fluid suspension of nanowires is provided to the substrate andelectrodes, the direction of the fluid flow will also aid in nanowirealignment. In suitable embodiments, the direction of the fluid flow willbe the same as, or substantially the same as, the direction of thee-field (though the fluid flow and the e-field can be in oppositedirections which still aids nanowire alignment parallel to the e-fieldas well as in different directions).

In the associated or pinned state, the nanowires are free to rearrange,migrate and/or align along the length of the electrodes. Nanowires thatare already substantially aligned with the e-field will tend to migratealong the electrode pair until contacting, and/or being repelled by, anearest neighbor. Nanowires that are not substantially aligned will tendto migrate such that they become aligned as they contact, and/or arerepelled by, nearest neighbor nanowires and, an equilibrium between thevarious forces acting on the nanowires is reached. The lateral mobility(i.e., along the electrode pairs, perpendicular to the e-fielddirection) of the nanowires allows them to accommodate a chronologicalsequence of alignment and association events without giving rise tonanowire clumping. That is, as nanowires are continuously supplied tothe electrode pairs (i.e., from a suspension) additional nanowires areable to associate with the electrodes, as the nanowires that arepreviously associated are freely mobile such that they move out of theway to accommodate additional wires.

FIG. 9 a shows a series of eleven electrode pairs, each of whichcomprises at least ten nanowires spanning the electrodes. In general,the nanowires are aligned substantially in the same direction and areall at approximately the same separation distance for a given electrodepair with little, if any, overlap or clumping of nanowires. FIG. 9 bshows a series of ten electrode pairs, each of which comprises a muchhigher density of nanowires (at least about 20-30 nanowires perelectrode pair). The density of nanowires that can be associated andultimately coupled to the electrodes depends upon the number ofnanowires provided (e.g., the concentration of nanowires in suspension)the size of the electrodes and the number of electrode pairs. Exemplarynanowire densities range from about 1 nanowire per 0.5 μm to about 1nanowire per 100 μm, a suitable density range is about 1-5 μm pernanowire.

Suitably, association of the nanowires with an electrode pair occurswhen an alternating electric current is generated at a frequency ofabout 5 Hz to about 5 kHz, suitably about 10 Hz to about 5 kHz, about 10Hz to about 2 kHz, about 10 Hz to about 1 kHz, about 100 Hz, about 200Hz, about 300 Hz, about 400 Hz, about 500 Hz, about 600 Hz, about 700Hz, about 800 Hz, or about 900 Hz (though other frequencies can also beused). Suitably, the amplitude of the AC electric field that isgenerated so as to associate the nanowires with the electrode pair isabout 0.1 V to about 5 V, suitably about 0.5 V to about 3 V, about 0.5 Vto about 2 V, about 0.6 V, about 0.7 V, about 0.8 V, about 0.9 V, about1.0 V, about 1.1 V, about 1.2 V, about 1.3 V, about 1.4 V, about 1.5 V,about 1.6 V, about 1.7 V, about 1.8 V, or about 1.9 V. As usedthroughout, when discussing amplitude values of electric fields, voltagevalues (V) represent peak-to-peak voltages (V_(p-p)). These ranges aresuitably used when associating nanowires that are CSS and/or CNOS incomposition.

Following association of the nanowires with the electrode pair,modulation of the energizing of the electrode pair is then used tocouple nanowires onto the electrode pair. Modulating the electrode pairsuitable comprises modulating an AC electric field between the electrodepair. As used herein the term “modulate” or “modulation” means to varyor adjust the energizing. For example, the frequency, amplitude, orboth, of an electromagnetic wave, electric field or electric signal canbe modulated modulated. The terms “vary” and “adjust” include increasingas well as decreasing the energizing (e.g., the field or signal). Forexample, modulation of the AC electric field can include frequencymodulation, amplitude modulation, as well combinations of both, whethertaking place at different times or at the same time. Modulation includesincreasing both the frequency and amplitude, increasing the frequencyand decreasing the amplitude, and decreasing the frequency andincreasing the amplitude, and decreasing both the amplitude and thefrequency. The timing of these increase and/or decreases can occur atthe same time or at different times.

Suitably, modulating the energizing (e.g, the AC electric field) betweenthe electrode pairs causes the nanowires to be come coupled or “locked”onto the electrode pair. “Coupling” or “locking,” as used herein, refersto a state in which the nanowires become so strongly attracted to theelectrodes that they do not continue to move or shift under continuedfrequency modulation or under flow conditions (generally moderate flow),and also remain in their aligned state. While not wishing to be bound bythe following theory, it is hypothesized that upon modulation of the ACfield, the equilibrium between opposing forces represented in FIG. 11(dielectrophoretic and electro-osmotic) is shifted such that thedielectrophoretic force becomes greater than the electro-osmotic force,and hence the nanowires are drawn closer to the electrodes than in the“association phase” described above. As the nanowires reach a criticaldistance from the electrodes, localized forces, such as van der Wallsforce, are strong enough to couple the nanowires to the electrodes. Asshown in FIG. 11, the nanowire 208, coupled or locked to the electrodepair (204, 205), is substantially immobile. FIG. 8 b shows a micrographof the same four electrode pairs as shown in FIG. 8 a, followingmodulation of the AC electric field (change from association phase tocoupling phase). Almost immediately after modulating the AC field to afrequency and/or amplitude at which nanowire coupling occurs, thenanowires attract to the electrodes and then become substantiallyimmobilized. As shown in FIG. 8 b, the ends of the wires that are incontact with the electrode surface, and now coupled to the electrodesvia van der Walls attractions (or some other force), appear to glow inthe micrograph (arrows in FIG. 8 b). Video of the process shows that thewires no longer are able to migrate, rearrange, align or otherwise moveas in the association phase in FIG. 8 a.

Modulating the energizing of the electrode pair, suitably modulating thefrequency of the AC electric field, includes increasing the frequency ofthe field (from that of the association phase above) to the range ofabout 1 kHz to about 500 kHz, suitably about 1 kHz to about 400 kHz,about 1 kHz to about 300 kHz, about 1 kHz to about 200 kHz, about 1 kHzto about 100 kHz, about 10 kHz to about 100 kHz, about 20 kHz to about100 kHz, about 30 kHz to about 100 kHz, about 40 kHz to about 100 kHz,about 50 kHz to about 100 kHz, about 60 kHz to about 100 kHz, about 70kHz to about 100 kHz, about 80 kHz to about 100 kHz, about 90 kHz toabout 100 kHz, or about 100 kHz. It should be understood that otherfrequency ranges above those described herein can also be utilized.Modulating the frequency also includes decreasing the frequency of theAC field, e.g., decreasing the frequency to about 1 Hz to about 10 Hz.

Modulating the amplitude of the AC electric field also includesincreasing the amplitude of the field (from that of the associationphase above) to the range of about 2 V to about 20 V, suitably about 2 Vto about 10 V, about 3 V, about 4 V, about 5 V, about 6 V, about 7 V,about 8 V or about 9V. It should be understood that other amplituderanges above those described herein can also be utilized. Modulating theamplitude also includes decreasing the amplitude of the AC field, e.g.,decreasing the amplitude to about 0.01 to 0.1 V.

In suitable embodiments of the present invention, the frequency and theamplitude of the AC field are modulated at the same time, orsubstantially the same time (within a few minutes of each other forexample). For example, both the frequency and the amplitude can beincreased from the values used during the association phase, therebycausing the nanowires to couple onto the electrodes. For example, thefrequency and amplitude ranges used in the association phase, e.g,between about 10 Hz and about 1 kHz, and between about 0.5 V and 2 V,can be increased to ranges where the nanowires couple onto theelectrodes, e.g., between about 1 kHz and about 100 kHz, and betweenabout 2 V and about 10 V. In suitable embodiments, the frequency andamplitude values utilized during the association phase, e.g., about 500Hz and 1 V, are modulated to about 10 kHz, or about 100 kHz, and about 4V.

In additional embodiments of the present invention, the frequency andamplitude can be modulated separately. That is, for example, thefrequency of the electric field can first be increased from a valueutilized during the association phase to a higher frequency, and thenthe amplitude of the electric field can be increased at some later time(or the amplitude can be modulated before the frequency). For example,the amplitude can be increased after about a few seconds, a few minutesor several minutes (e.g, 5, 10, 20, 30, 40 minutes) after the frequencyis increased. Suitably, the frequency and amplitude ranges used in theassociation phase, e.g, between about 10 Hz and about 1 kHz, and betweenabout 0.5 V and 2 V, can be increased to ranges where the nanowirescouple onto the electrodes, e.g., between about 1 kHz and about 100 kHz,and between about 2 V and about 10 V.

In a further embodiment of the invention, modulation of the frequencybefore modulating the amplitude can be used to further align thenanowires prior to the ultimate coupling onto the electrodes. Forexample, the frequency and amplitude ranges used in the associationphase, e.g, between about 10 Hz and about 1 kHz, can first be modulatedby only increasing the frequency, for example to from about 10 kHz toabout 100 kHz, while maintaining the amplitude at the association phaselevel, for example between about 0.5 V and 2V, suitably about 1 V. Ithas been determined that by maintaining the amplitude of the electricfield at a relatively low amplitude (e.g., 1 V peak-to-peak) and thenmodulating the frequency from about 500 Hz up to about 100 kHz,nanowires that are associated with electrodes will migrate and rearrangesuch that in some cases they attain better alignment than in theassociation phase (i.e., less wires crossing or at angles that are notparallel to each other).

Modulating (e.g., increasing) the frequency in this manner has also beennoted to uncross or untangle nanowires that may have been crossed or incontact with each other in the association phase. Provided that there issufficient space available on the electrode surface (i.e., the densityis not exceedingly high), the nanowires are able to uncross andrearrange such that a greater number of nanowires are parallel aftermodulation, than prior to the modulation. For example, as shown in FIGS.10 a and 10 b, nanowires that were initially crossed in FIG. 10 a(arrows) are able to migrate and rearrange during the modulationalignment phase such that they are now uncrossed in FIG. 10 b. Thisphase is termed “modulation alignment” throughout and is an alignmentphase in addition to the alignment that takes place during theassociation phase. It should be understood that the modulation alignmentphase is not critical to the practice of the present invention and canbe omitted such that the methods of the present invention proceed fromthe association phase directly to the coupling phase.

After the nanowires have been aligned sufficiently during the modulationalignment phase (if used) the amplitude is then modulated so as to causethe wires to couple onto the electrodes (see FIG. 10 c, note intensebrightness at nanowire tips where nanowires are coupled onto theelectrodes). It should be noted that while additional frequencymodulation is not necessary after the modulation alignment phase,additional modulation of the frequency can be used in order to cause thenanowires to couple onto the electrodes. In suitable embodiments, theamplitude of the electric field is increased from the range used duringthe association phase (and the modulation alignment phase), e.g.,originally between about 0.5 V and about 2 V, to the amplitude rangeutilized for nanowire coupling e.g., increased to about 2 V to about 10V, suitably about 4 V.

While the modulation frequencies and amplitudes described above can beutilized on any nanowire composition/structure, suitably they are usedwhen positioning nanowires that are core:shell:shell (CSS) and/orcore:no oxide:shell (CNOS) in composition. In cases where CSS and/orCNOS nanowires are utilized, the outermost shell layer is suitably ametal or other material which possesses a surface charge (eitherpositive or negative). In solution, the outer (metal) shell attractsoppositely charged ions creating an electric double layer on the surfaceof the nanowire. The presence of a surface charge on the nanowires aidsin association and coupling as described in the theory section above.The charge layer that forms on the nanowires also reduces, limits oreliminates nanowire crossing and/or clumping, as the nanowires tend torepel each other when brought in close proximity. As the nanowires arebeing associated/aligned with the electrodes, the lateral mobility ofthe nanowires, along with a repulsive surface charge, providessufficient movement such that crossed nanowires are able to uncross anddistribute themselves along the electrode (see, e.g., FIGS. 10 a and 10b).

When positioning nanowires that comprise a core:shell (CS) structure(e.g., SiO₂), higher frequency and/or amplitude AC electric fields maybe required to associate and couple nanowires onto electrodes. Forexample, nanowire association may require generation of an AC field witha frequency of about 1 kHz to about 50 kHz, more suitably about 5 kHz toabout 20 kHz, or about 10 kHz, and an amplitude of about 1 V to about 10V, suitably about 2 V to about 5 V, or about 2 V. Nanowire coupling ofCS nanowires may require modulation of the AC field to a frequency ofabout 50 kHz to about 500 kHz, suitably about 75 kHz to about 200 kHz,or about 100 kHz, and amplitude modulation to about 3 V to about 10 V,suitably about 3 V to about 5 V, or about 4 V. Nanowires that do notcomprise an outer (metal) shell layer, and therefore an outer chargedlayer, in general may require generation of higher frequencies and/oramplitudes to achieve association and/or coupling, as compared tonanowires that comprise an outer metal shell layer (i.e., CSS).

Thus, in suitable embodiments, the present invention provides methodsfor separating one or more conductive nanowires (e.g., CSS conductingnanowires) from a mixture of conductive and semiconductive (e.g., CSsemiconductive) nanowires. As noted above, CSS nanowires suitablycomprise an outer shell or of metal, thereby making these wiresconductive. Thus, in suitable embodiments, a solution comprising one ormore conductive nanowires and one or more semiconductive nanowires isprovided proximate to an electrode pair. The electrode pair is thenenergized, whereby the conductive and semiconductive nanowires becomeassociated with the electrode pair. Then, the energizing is modulated,whereby the conductive nanowires become coupled onto the electrode pair.The semiconductive nanowires are then removed.

Suitable methods for generating an AC field between the electrode pairsare described throughout, including a direct electrical connection andan electromagnetic wave. Exemplary AC electric fields useful in theassociation phase include AC fields having a frequency of about 10 Hz toabout 5 kHz and an amplitude of about 0.5 V to about 3 V. Modulating theAC electric field by increasing the frequency to about 1 kHz to about500 kHz, but maintaining or increasing the amplitude of the AC field toabout 1 V to about 4 V preferentially pins and locks the conductivenanowires (e.g., CSS nanowires), while semiconductive nanowires are notcoupled (locked). Subsequent removal of uncoupled, semiconductivenanowires, provides a method for selectively removing conductivenanowires from suspension. Thus, the conductive nanowires that arecoupled to the electrodes can be utilized in various applications, andsimilarly, semiconductive nanowires remaining in the solution (nowsubstantially free of conductive nanowires) can also be utilized inadditional applications.

In further embodiments, alignment, association and coupling can beperformed using nanowires that are n-doped on one end and p-doped on theopposite end. The use of two different dopings results in nanowires thatwill have two induced dipoles when an electric field is applied. Aselectrons are more mobile in n-doped materials as compared to p-dopedmaterials, the n-doped “end” of a nanowire will have a stronger dipolethan the p-doped “end.” Consequently, the differences in doping of thenanowires allows for alignment and deposition in a predetermineddirection. For example, a set of electrodes can be used (e.g., three orfour electrodes, though more electrodes can be used) in which one pairof electrodes is energized at a higher level than a second pair.Suitably, pairs of electrodes in an electrode set are positioned suchthat one electrode pair is in the same plane as the other pair, forexample next to each other or one pair above another pair (e.g.,positioned in line with each other on the substrate). Nanowires aredrawn toward the electrode set as noted throughout. However, due to thehigher electric field between two of the electrodes, the n-doped end ofthe nanowires tends to associate and couple with these electrodes, whilethe p-doped end of the nanowires associates and couples with the lowerelectric field pair. In this way then, nanowires can be aligned inpredetermined directions such that substantially all of the nanowires(e.g., greater than 50%, greater than 60%, greater than 70%, greaterthan 80%, greater than 90%, and suitably about 100% of the nanowires)align in the same orientation and direction (i.e., n-doped ends allpointing the same direction). Alignment and deposition performed in thismanner is especially useful when preparing arrays of nanowires, forexample, for use as diodes, where all of the n-doped ends of nanowiresare positioned together on one side of an electrode set.

In embodiments in which a waveguide or similar instrument is used togenerate an electric field, nanowire alignment, association and couplingcan occur within a single step. That is modulation of the frequencyand/or amplitude is not required (though it can be used) to achievenanowire coupling. For example, when a waveguide is utilized,frequencies on the order of about 1 GHz to about 5 GHz are utilized togenerate an electric field. Suitably, the frequency is about 2 GHz toabout 3 GHz, or about 2.3 GHz to about 2.5 GHz, for example about 2.45GHz. The amplitude of the electric field generated by the waveguide issuitably on the order of about 1 V to about 10 V. Generation of anelectric field at frequencies of this magnitude cause nanowires toalign, associate and couple at substantially the same time. Upongeneration of an electric field using a waveguide, nanowires arealigned, associated and coupled onto the electrode pairs, suitably inone nearly continuous step or motion. A separate modulation of thefrequency and/or amplitude is therefore not required (though modulationscan be used) to couple the nanowires onto the electrodes.

In additional embodiments, a nanowire suspension is simply placed on topof electrode pairs 207, without a flow of the suspension, and thus thenanowires are in a stationary suspension prior to alignment anddeposition. Upon application of an electric field, nanowires associateand couple as described throughout. However, due to the lack of fluidflow conditions, the nanowires are not pre-aligned prior to deposition.This allows for nanowire deposition in directions that are normal to oneanother (i.e., in both the x and y directions). For example, as shown inFIG. 12, nanowires can be aligned and deposited in orientations that areperpendicular to one another, simply by providing electrode pairs in thedesired orientation. In addition to deposition in x and y directions,nanowires can be deposited an any direction or orientation. Theseembodiments are particularly useful in electric device constructionwhere it is often desired to have wires in several differentorientations, including in orientations perpendicular to each other.

In a further embodiment, placement of one or more metallic elementsbetween electrode pairs can be used to enhance or aid nanowire alignmentand deposition. For example, as shown in FIG. 13, depositing orpositioning one or more metallic elements 1302, for example metallicstrips, between electrodes 204 and 205 of an electrode pair, aids innanowire 208 alignment on the electrodes. Any suitably metal can be usedin the methods of the present invention, for example, Al, Cu, Fe, Au,Ag, etc., or combinations thereof. FIG. 13 shows that nanowires alignand deposit in a substantially straight and aligned fashion, eachdepositing between a pair of metal elements. By selecting the size andorientation of metallic elements 1302, the spacing between nanowires canbe controlled and tailored such that the association and coupling ofsubstantially evenly spaced, parallel, aligned nanowires can beachieved.

The present invention also provides methods for positioning one or morenanowires. In such methods, one or more nanowires are provided proximateto an electrode pair. The electrode pair is then energized, whereby thenanowires become associated with the electrode pair, wherein one or moremetallic elements are positioned between electrodes of the electrodepairs, such that inter-nanowire distances between adjacent associatednanowires vary by less than about 50% of a standard deviation. Infurther embodiments, the methods further comprise modulating theenergizing between the electrode pairs, whereby the nanowires becomecoupled onto the electrode pair, and wherein one or more metallicelements are positioned between electrodes of the electrode pairs, suchthat inter-nanowire distances between adjacent coupled nanowires vary byless than about 50% of a standard deviation. See e.g., FIG. 13. Thepresent invention therefore provides methods for positioning nanowiressuch that the inter-nanowire distances between adjacent nanowires can becontrolled to such a degree that the distances vary less than about 50%of a standard deviation from the mean of the distances. As used herein,adjacent nanowires refers to associated and/or coupled nanowires thatare positioned next to each other with no other nanowires between them.As used herein, inter-nanowire distances refers to the distance betweenadjacent nanowires. As discussed herein with reference to inter-nanowiredistances, a standard deviation indicates the standard deviation of themean (average) of the inter-nanowire distances. The standard deviationof the mean of inter-nanowire distances can be readily calculated, byfirst calculating the mean of the inter-nanowire distances (Sum ofinter-nanowire distances/number of distances). The standard deviation ofthe mean (σ) is then calculated as:

$\sigma = \sqrt{\frac{1}{N}{\sum\limits_{i = 1}^{N}\; \left( {x_{i} - \overset{\_}{x}} \right)^{2}}}$

where x_(i) represents each of the individual inter-nanowire distances,x-bar is the mean of the inter-nanowire distances, and N is the totalnumber of inter nanowire distances.

By utilizing the methods of the present invention incorporating metallicelements positioned between the electrodes, the inter-nanowire distancesbetween adjacent nanowires can be controlled such that the variationbetween these distances will be less than about 50% of a standarddeviation from the mean, for example, less than about 40%, less thanabout 30%, less than about 20% or less than about 10% of a standarddeviation from the mean.

The present invention also provides substrates comprising at least afirst pair of electrodes and at least four nanowires coupled between thefirst pair of electrodes, wherein the inter-nanowire distances betweenadjacent coupled nanowires varies by less than about 50% of a standarddeviation. In suitable embodiments, the substrates further comprisethree or more metallic elements positioned between the electrodes of thefirst electrode pair. See e.g., FIG. 13. In suitable embodiments, thesubstrates comprise at least four nanowires coupled between a first pairof electrodes, wherein the inter-nanowire distances between adjacentcoupled nanowires varies by less than about 50% of a standard deviationfrom the mean, for example, less than about 40%, less than about 30%,less than about 20%, or less than about 10% of a standard deviation fromthe mean.

In a still further embodiment, one of the electrodes of an electrodepair can comprise two or more electrodes in the same plane (i.e., asplit electrode configuration), each of which is connected to a separatesource of an AC field. In these embodiments, an AC field can begenerated between both electrodes of the split electrode configurationand the other electrode of the pair in such a way that the AC field atone of the electrodes of the split electrode configuration is modulatedto provide nanowire association and coupling, while the other electrodeis not modulated. In such a configuration, substantially straightnanowires of a preferred length associate and couple between themodulated electrode of the split configuration and the other electrodeof the pair, while the non-modulated electrode attracts stray, undesirednanowires, thus removing them from the alignment process. In this way,substantially parallel and uniform nanowire deposition can be achieved.

Following coupling of the nanowires onto the electrodes, uncouplednanowires can then be removed from the electrode pairs so as tosubstantially eliminate nanowires that are not fully aligned, not fullycoupled, overlapped, crossing, or otherwise not ideally coupled to theelectrode pair. Nanowires that are to be removed following the couplingphase are described herein as “uncoupled nanowires.” Any suitable methodfor removing uncoupled nanowires can be used. For example, the uncouplednanowires can be removed using tweezers (e.g., optical tweezers, see,e.g., U.S. Pat. Nos. 6,941,033, 6,897,950 and 6,846,084, the disclosuresof each of which are incorporated herein by reference in theirentireties) or similar instrument, or by shaking or physicallydislodging the uncoupled nanowires. Suitably, uncoupled nanowires areremoved by flushing away the nanowires.

As used herein, the term “flushing away” includes processes where afluid (either gaseous or liquid phase) is flowed over or around thenanowires so as to remove them from the electrode pairs. Flushing awayalso includes translating or moving the electrode pairs so as to createa flow of fluid over the nanowires. In suitable embodiments, a liquid isflowed over the nanowires so as to flush away uncoupled nanowires.Flowing a liquid over the nanowires includes simply the application of aliquid to the nanowires as well as causing a liquid to move over thenanowires with a velocity such that uncoupled nanowires are flushedaway. Fluid velocities for flushing away uncoupled nanowires can begenerated using any method known in the art, including, but not limitedto, gravity, a nozzle or spray apparatus, a suction apparatus and thelike. As shown in FIG. 8 c, suitably the direction of fluid flow isparallel to the nanowires, and hence perpendicular to the electrodepairs, though any direction of fluid flow can be used. As shown in FIG.8 c, uncoupled nanowires (208) that were either not completely coupledto the electrodes are washed away in the fluid flow. The arrows in FIG.2 represent an exemplary direction of fluid flow when utilizingapparatus 200. It should be understood that the removal of uncouplednanowires, while designed to remove many if not all uncoupled nanowires,may in fact leave some uncoupled nanowires still contacting theelectrode pairs. The ordinarily skilled artisan will readily understandremaining uncoupled nanowires will not significantly impair further useof coupled nanowires. Any suitable liquid can be used to flush awayuncoupled nanowires, for example, a solvent such as IPA, water or otheraqueous solvent and the like. Suitably the solvent is the same solventthat originally contained the nanowires, just without additionalnanowires present in the suspension. Flushing of the nanowires can beperformed in any direction relative to the nanowires (i.e., parallel,perpendicular, or other orientation). As the nanowires are coupled ontothe electrodes and therefore fixed in that position, fluid flowconditions do not disturb that coupling, even if the flow is normal tothe plane of nanowire alignment.

In addition to removing one or more uncoupled nanowires from theelectrode pairs, the nanowires are also suitably dried following thecoupling phase. Generally, drying will occur after the removal ofuncoupled nanowires, but it is not necessary to remove uncouplednanowires prior to drying. Drying of the nanowires can occur via anysuitable process known in the art, for example, air drying (whetherstagnant or moving) to allow for evaporation, heating using an oven orother suitable device, or other mechanism. FIG. 8 d shows a micrographof coupled nanowires following a drying process.

FIG. 14 a represents a flowchart 1400 showing a method of nanowirealignment and deposition in accordance with one embodiment of thepresent invention. In step 1402 of flowchart 1400, a channel, forexample channel 206 as represented in FIG. 2, is first filled with asuitable solvent, for example, IPA. It should be noted that the initialfilling of the channel is not required. In step 1404, the channel isthen filled with a nanowire suspension, for example a nanowire ink. Asnoted throughout, it is not necessary to utilize a channel, but ratherthe nanowire suspension can simply be placed directly on the electrodepairs. The nanowires are then associated or pinned to the electrodes instep 1406, suitably as set forth in the association phase as describedthroughout. En suitable embodiments, the electric field is thenmodulated so as to align the nanowires 1408. As described throughout,suitably the alignment modulation phase comprises increasing thefrequency to between about 10 kHz and about 100 kHz to allow nanowiresthat are crossed or not optimally aligned to migrate and align on theelectrodes. As discussed throughout, alignment modulation phase, step1408, is not required and can be omitted from the methods of the presentinvention.

Following alignment modulation (or association if alignment modulationis not utilized) the nanowires are then coupled or locked onto theelectrode pairs in step 1410, by modulating the electric field asdescribed throughout. Suitably, the electric field is modulated byincreasing both the frequency and the amplitude from the associationphase. However, in embodiments where a modulation alignment phase isused, suitably only the amplitude of the electric field is increased,though the frequency can also be increased (or decreased if desired).

Uncoupled nanowires are then removed or released, in step 1412, from theelectrode pairs using any of the methods described herein or otherwiseknown in the art. Suitably, nanowires are released or removed from theelectrode pairs by flowing a liquid (e.g., IPA) over the nanowires.

After uncoupled nanowires have been removed from the electrode pairs, instep 1414, a decision analysis is made where it is determined whether asufficient number or sufficient density of nanowires has been achievedby the alignment and deposition methods of the present invention (i.e.,steps 1402-1412). This decision analysis can be made by inspecting theelectrode pairs in any manner, for example, via visual inspection(microscope or other suitable device), or by using an electrical orother signal to monitor the number and/or density of nanowires at anelectrode pair. A “sufficient number of nanowires” can be a pre-set orpre-determined number of nanowires, a number of nanowires that isdetermined at the time of deposition, or a number of nanowires that isdependent upon the electrical or other of characteristics of the wires.For example, as discussed throughout, a “sufficient number of nanowires”can be determined by measuring the impedance, capacitance, resistance orother characteristics of the nanowires coupled onto the electrode pairs.

If it is determined that a sufficient number of nanowires has beencoupled onto the electrode pairs, decision analysis step 1414 willreturn an answer of “yes,” in step 1416, following which, a finalnanowire flush step 1420 will suitably begin. It should be noted thatnanowire flush step 1420 is not required. Following this nanowire flush(or following a “yes” decision in step 1416), the nanowires are thendried in step 1422.

If is it determined that a sufficient number of nanowires has not beencoupled onto the electrode pairs, decision analysis step 1414 willreturn an answer of “no,” in step 1418. An answer of “no” means that asufficient number of nanowires have not been deposited on at least oneof the electrode pairs. Therefore, in order to provide for additionalnanowire coupling, steps 1406 through 1412 are repeated. It should benoted that additional nanowires can also be provided, for example, viaintroduction of a nanowire suspension as in step 1404. Followingalignment and deposition of an additional population of nanowires, asecond, decision analysis, step 1414, is performed. At the end of thisdecision analysis, a sufficient number of nanowires will have eitherbeen deposited (an answer of “yes”), and the steps 1416, 1420 and 1422will follow, or a sufficient number of nanowires will not have beendeposited (an answer of “no”), and steps 1406-1412 are repeated. Thistype of feedback loop can be repeated as many times as necessary (e.g.,2, 3, 4, 5, 10, 15, 20, 50, 100 times, etc.) until a sufficient ordesired number of nanowires are coupled onto the electrode pairs.

FIGS. 14 b and 14 c represent exemplary nanowire alignment anddeposition sequences in accordance with embodiments of the presentinvention. For all plots, the x axis represents a series of steps in anexemplary alignment and deposition process, for example, correspondingto steps 1402-1422 as shown in FIG. 14 a. It should be noted that theamount of time between successive steps is not represented by thedistance between the steps as some steps may occur soon after eachother, while others may be separated by longer times. In FIG. 14 b, theupper plot represents the voltage and frequency of the AC electric fieldat each of the process steps. As represented in FIG. 14 b, suitably theelectric field is off (i.e., amplitude and frequency are zero, or verylow) during the channel fill steps (1402 and 1404, deposition steps 0and 1) when a solvent and/or a nanowire suspension are added to theelectrode pairs (e.g., in channel 206 as shown in FIG. 2).

Upon initiation of the association phase, represented by step 1406 anddeposition step 2 in FIG. 14 b, both the amplitude and the frequency ofthe AC electric field are turned on/increased such that nanowireassociation and alignment begin. Suitable amplitudes and frequencies foruse during the association phase are described herein. For example, asrepresented in the upper graph of FIG. 14 b, suitably the amplitude(voltage V) of the electric field is about 0.5 V to about 2 V, forexample, about 1 V. The frequency of the electric field as representedin the upper graph of FIG. 14 b is suitably about 100 Hz to about 1 kHz,for example, about 500 Hz.

As the deposition sequence moves to step 3, the modulation alignmentphase represented as step 1408 in FIG. 14 a, the frequency of the ACfield is modulated, e.g., increased. As discussed throughout, suitably,during the modulation alignment phase, only the frequency of the fieldis increased, though the amplitude can be increased as well. Suitably,the frequency of the field is increased to between about 10 kHz andabout 100 kHz (e.g., 10 kHz as shown in FIG. 14 b), while maintainingthe amplitude at the association phase level, for example between about0.5 V and 2V, suitably about 1 V.

Moving to deposition step 4, the coupling or locking phase (representedas step 1410 in FIG. 14 a), the amplitude of the AC electric field ismodulated so as to cause the nanowires to couple onto the electrodes. Asdiscussed throughout and represented in FIG. 14 b, suitably theamplitude of the AC field is increased, while maintaining the frequencyat the previous level (though the frequency can be increased as well).Suitably, the amplitude of the AC field is increased to between about 2V and about 10 V, for example, to about 4V, so as to initiate thenanowire coupling phase.

Following nanowire coupling, the AC field is suitably turned off. Forexample, the frequency and the amplitude of the field are reduced tozero, or to substantially low values. This allows for removal ofuncoupled nanowires (step 1412). A determination can then be made if asufficient number of nanowires have been deposited (step 1414), prior todetermining whether the alignment and deposition process needs to berepeated. If it is determined that there are sufficient nanowiresdeposited, the nanowires can then be dried.

The bottom plot in FIG. 14 b represents the flow of solvent (e.g., IPA)and nanowire suspension (NW Ink) in arbitrary units during thedeposition process. The deposition steps along the x-axis correspond tothe same steps as described above with regard to the upper graph in FIG.14 b. Initially, during the channel fill step 0, step 1402, no nanowiresuspension is introduced, and only solvent is flowing over theelectrodes, though at a higher velocity/volume relative to later steps.Upon introduction of the nanowire suspension in step 1, the flow of thenanowire suspension is increased, and the flow of solvent is decreased,so as to allow for nanowire association during step 2, while maintaininga fluid flow to aid in nanowire alignment. After step 2, the nanowiresuspension is shut off, leaving only the flow of solvent through steps2-4, association (step 1406), modulation alignment (step 1408) andcoupling (step 1410). Following the completion of coupling step 5 (step1410), the flow of solvent is increased so as to aid in removal ofuncoupled nanowires in step 6 (step 1412). This elevated flow ismaintained during the decision analysis process (step 1414), and uponcompletion of all nanowire alignment and deposition, increased furtherin step 7 (step 1420) to finally flush the electrodes prior to drying instep 8 (step 1422).

FIG. 14 c shows the sequence of an additional nanowire depositionprocess in accordance with an embodiment of the present invention. Thesequence of amplitude and frequency modulation (upper graph) is the sameas that in FIG. 14 b. The flow of solvent and nanowire suspension,however, represents one exemplary alternative to that shown in FIG. 14b. In the lower graph of FIG. 14 c, the sequence of flow of the nanowiresuspension is the same as that in FIG. 14 b. However, the flow ofsolvent has been increased by a factor of about 2 (flow is representedin arbitrary units) for steps 2-6 as compared to FIG. 14 e. It should benoted that FIG. 14 c simply represents an additional embodiment of thepresent invention in which the flow of solvent is increased. It shouldnot be construed that the increase in flow must be on the order oftwo-fold, as more or less flow can be used.

In a still further embodiment, the present invention provides one ormore electrodes comprising one or more nanowires positioned according tothe methods described herein. Suitably, the electrodes compriseelectrode pairs, and each electrode pair comprises a plurality ofnanowires (e.g., more than 2, more than 5, more than 10, more than 20,more than 50 or more than 100 nanowires, etc.) coupled or pinned to theelectrodes. As discussed throughout, the methods of the presentinvention allow for the alignment and deposition of nanowires onelectrode pairs such that substantially all of the nanowires that aredeposited are substantially parallel to each other and relatively evenlyspaced. This aids in use of the nanowires in devices and helps withtransferring nanowires to additional substrates or device contacts.

In a further embodiment of the present invention, nanowires that arecoupled onto the electrodes can then be transferred onto a substrate.Suitably, the nanowires have been dried prior to the transfer, thoughnanowires that have not been dried can also be transferred. As usedherein, the term “transfer” means to move or relocate the nanowires fromthe electrodes on a transfer substrate to a receiving substrate.Receiving substrates that can be utilized in the transfer methods of thepresent invention include any suitable material, for example,semiconductor, dielectric material, etc. Suitably the receivingsubstrate utilized in the transfer methods comprises one or more deviceelectrode or other suitable contact onto which nanowires are to betransferred (e.g., drain, gate or source electrodes). For example, asshown in FIG. 15, nanowires 208 that have been previously coupled to anelectrode pair (204, 205) on a transfer substrate 1508 can betransferred to a receiving substrate 1502 which comprises additionalcontacts or electrodes (1504, 1506) patterned or positioned on itssurface. Single nanowires can be transferred one at a time (see e.g.,dotted line in FIG. 15), or multiple nanowires can be transferred fromelectrodes to the substrate/contacts.

In suitable embodiments, the transfer methods of the present inventionprovide a method for “printing” nanowires onto a transfer substratewhich comprises one or more contacts or electrodes. For example, atransfer substrate 1508 comprising one or more electrode pairs (204,205) which comprises one or more nanowires 208 can be used in effect asa type of “print” head. That is, one or more nanowires that are coupledonto the electrodes (e.g., using the methods and processes describedthroughout) on the transfer substrate can be transferred to a suitablecontact (1504, 1506) on a receiving substrate 1502 by simply positioningthe transfer substrate (or the receiving substrate can be positioned)such that the nanowires can be transferred from the electrode pair tothe contact. For example, the transfer substrate can be positioned abovea receiving substrate which comprises a contact and then the nanowiresbrought in position relative to the contact such that the wires transferfrom the electrodes to the contact. This process can be repeated as manytimes as desired, relocating the transfer substrate in relation to thecontacts on the receiving substrate such that nanowires can betransferred to a variety of different locations on the receivingsubstrate. Single nanowires or a plurality of nanowires can betransferred from the electrodes to the contacts. As such then, thetransfer methods of the present invention provide a type of printing inwhich nanowires are transferred from electrodes to contacts in a precisemanner.

The present invention also provides methods for positioning one or morenanowires on a substrate. Suitably the methods comprise providing one ormore nanowires in a suspension (e.g., as a nanowire ink) and energizingthe electrode pair, whereby the nanowires become associated with theelectrode pair (i.e., an association phase). In exemplary embodiments,the energizing of the electrode pair comprises generating an alternatingcurrent (AC) electric field between an electrode pair on a transfersubstrate. Suitable AC field characteristics for use in the associationphase of the transfer methods of the present invention are describedthroughout. The energizing of the electrode pair is then modulated,whereby the nanowires become coupled onto the electrode pair (i.e., acoupling phase). For example, an AC electric field between the electrodepair(s) is modulated. Suitable AC field modulations for use in thecoupling phase of the transfer methods of the present invention aredescribed throughout. Uncoupled nanowires are then removed from theelectrodes, and then the coupled nanowires are transferred onto thesubstrate, suitably onto contacts or electrodes on the receivingsubstrate. The present invention also provides substrates comprising oneor more nanowires positioned according to the methods of the presentinvention. As described throughout, the methods of the present inventionallow for the alignment and deposition of nanowires such thatsubstantially all of the nanowires are parallel to each other. Thisallows for increased ease of transfer of the nanowires to a finalsubstrate and/or device contact.

In additional embodiments, the substrate comprising the electrodes andcoupled nanowires can be utilized as a device substrate. For example,the electrodes themselves can be the contacts that will ultimately beused in a final device configuration. In other embodiments, theelectrodes can be etched away using suitable etchants known in the art(e.g., mildly alkaline ferricyannide-based etchant formulations that aregenerally commercially available), so as to remove the electrodes andleave behind the aligned, oriented nanowires. This etching can beperformed prior to any nanowire transfer or after the nanowires havebeen transferred to a transfer substrate/contact so as to leave onlynanowires and little or no residual electrode material.

In a further embodiment, the present invention provides methods forcontrolling the number of nanowires positioned on an electrode pair. Insuitable embodiments, the methods comprise positioning one or morenanowires according to the various methods of the present invention. Asignal is then applied to the electrode pair and the signal ismonitored. The positioning of nanowires on the electrode pair is thenstopped when the signal attains a pre-set value. By monitoring anelectrical signal at an electrode pair, the number of nanowires can becontrolled. Thus, once a pre-determined signal is obtained, thedeposition processes of the present invention can be stopped.

FIGS. 16 a and 16 b represent schematics showing apparatuses formonitoring the deposition of nanowires in accordance with suitableembodiments of the present invention. Apparatus 1600 represented in FIG.16 a comprises a device under test (DUT) 1602 which represents one ormore electrode pairs that have been involved and/or are involved in thealignment and depositions processes/methods of the present invention.Suitably, DUT 1602 will comprise a plurality of electrode pairs, each ofwhich can be monitored separately using the methods of the presentinvention.

Apparatus 1600 further comprises a signal generator 1604, which providesan electronic signal to DUT 1602. Suitably, signal generator 1604 is awaveform generator, and the same source for the AC electric field beingused in the association, modulation alignment and coupling phasesdescribed throughout. A signal that is generated by signal generator1604 is first applied to DUT 1602, for example, by applying a waveformto one electrode of an electrode pair (or several electrodes, each ofwhich is one electrode of a plurality of electrode pairs). The opposingelectrode of an electrode pair (or electrode pairs) (i.e., the electrodethat is not connected to signal generator 1604), is connected in seriesto a load resistor (R_(L)) 1606. The signal that is propagated throughthe DUT passes through the load resistor 1606 and then returns to thesignal generator. The signal passing through load resistor 1606 ismonitored by lock-in analyzer 1608, which comprises a device to monitorthe signal (e.g., an oscilloscope), to determine the characteristics ofthe signal that has passed through the DUT. For example, the frequency,amplitude, phase shift, etc. of the signal can be monitored, for exampleby using an oscilloscope. Lock-in analyzer 1608 suitably compares thesignal at load resistor 1606 to a reference signal provided by signalgenerator 1604. When a pre-determined signal is observed/measured atlock-in analyzer 1608, the apparatus of the present invention provides amechanism for stopping the deposition of nanowires. This mechanism caneither be monitored via human intervention, or can be set up to bemonitored electronically or automatically via computer, so that when thepre-determined value is reached, the deposition is stopped. Theembodiments of the present invention that provide a method forcontrolling the number of nanowires aligned and deposited utilizing apre-set signal value are described throughout as “active” monitoring andcontrol.

In suitable embodiments of the present invention, the signal that ismonitored at the DUT (i.e., at load resistor 1606) is a type of signalthat will change or vary as additional nanowires are aligned anddeposited (coupled) onto an electrode pair (the DUT). Examples ofsuitable signal types that can be monitored using the methods, systemsand apparatus of the present invention include, but are not limited to,impedance, voltage, capacitance and current, basic and complex waveformsand the like, as would be apparent to a person of ordinary skill in theart.

In further embodiments, the signal that is measured across load resistorR_(L) is fed to an analog-to-digital converter, and the digital signalis then amplified by a computer, digital signal processor or the like.This signal is then monitored as above to determine when a desirednumber of nanowires have been deposited.

For example, as nanowires are deposited/coupled onto an electrode pair,the impedance of the signal at the electrodes changes as more and morenanowires are coupled onto the electrodes. One skilled in the art willreadily understand that the impedance at a pair of electrodes changes asnanowires are deposited and hence connect the two electrodes. The realand imaginary portion of the impedance can therefore be monitored, suchthat a change in one or the other (or both) (i.e., the real or imaginaryportion of the impedance) signals that an additional nanowire(s) hasbeen coupled onto the electrode pair. More specifically, for example,the signal that is monitored by lock-in analyzer 1608 is the imaginaryportion of the impedance at DUT. Therefore, monitoring the impedance atload resistor 1606 (and hence, at DUT 1602) provides a method fordetermining, and therefore controlling, the number of nanowires that aredeposited at an electrode pair.

As the signal at the DUT is monitored, a pre-set or pre-determined(e.g., threshold) value can be chosen such that when such value isreached, apparatus 1600 provides a signal or other feedback that arequired or desired number of nanowires has been coupled onto theelectrodes. For example, a pre-determined impedance value can be set,such that when a sufficient number of nanowires have been deposited, theimpedance value (e.g., the imaginary part of the impedance) at the DUTattains, passes or closely approaches the pre-determine value. At thistime, the lock-in analyzer, reaching such a pre-determined value,provides some type of feedback or signal indicating that such value hasbeen obtained. In accordance with the present invention, the nanowiredeposition process is suitably stopped once this pre-determined value isreached. Stopping the nanowire deposition process can be achieved usingany suitable method, for example, by reducing the electric field betweenthe electrode pair, thereby stopping the positioning of the nanowires onthe electrode pair (in embodiments where the methods of the presentinvention are being utilized to align and deposit nanowires). Othersuitable methods to stop nanowire deposition include, removing thesource of nanowires (e.g., a nanowire suspension), removing theelectrodes from the source (e.g, by pulling the substrate out of thesuspension) or by other suitable methods dependent upon the depositionmethod being utilized.

FIG. 16 b represents an additional apparatus 1620 in accordance with thepresent invention. As with apparatus 1600 represented in FIG. 16 b,apparatus 1620 also comprises a device under test (DUT 602) (e.g., oneor more electrodes or electrode pairs), a signal generator 1604, a loadresistor 1606 and lock-in amplifier 1608. Apparatus 1620 also furthercomprises resistors R₁, 1622 and R₂, 1626. Resistor R₁ is suitablyplaced in parallel with DUT 1602, and in series with R₂, by positioningswitch 1624 in the proper orientation. A signal is generated by signalgenerator 1604 such that the signal is transmitted to both electrodes ofan electrode pair (represented by DUT 1602).

As in FIG. 16 a, the signal that is monitored at load resistor R_(L) isused to determine when a desired or required number of nanowires hasbeen deposited on the electrodes. R₂ can be either included in themonitoring loop or can be bypassed, for example, but moving switch 1624to the position where R₂ is removed from the loop. As in FIG. 16 a, asnanowires are deposited/coupled onto an electrode pair, the impedance ofthe signal (or other characteristic of the signal) changes as more andmore nanowires are coupled onto the electrodes. The real and imaginaryportion of the impedance can therefore be monitored, such that a changein one or the other (or both) (i.e., the real or imaginary portion ofthe impedance) signals that an additional nanowire(s) has been coupledonto the electrode pair. Suitably the signal that is monitored bylock-in analyzer 1608 is the imaginary portion of the impedance at DUT.Therefore, monitoring the impedance at load resistor 606 (and hence, atDUT 1602) provides a method for determining and therefore controllingthe number of nanowires that are deposited at an electrode pair.

The apparatuses presented in FIGS. 16 a and 16 b are referred throughoutas “active” monitoring systems of apparatuses. As the systems requiremonitoring by the lock in amplifier, followed by some response in orderto stop or halt nanowire deposition, the systems are therefore activemonitoring systems.

In additional embodiments, the present invention also provides for“passive” monitoring systems and apparatuses. In passive systems, ratherthan utilizing a lock-in amplifier or other system to actively monitorthe signal at the electrodes (and/or actively respond when a sufficientnumber of nanowires have been deposited), the apparatus is designed suchthat a resistor is placed in parallel with the DUT, e.g., R₁ in FIG. 16b. As nanowires are coupled on the electrode pairs, the resistanceacross the DUT drops below the threshold resistance value of R₁. Theelectric field at the DUT (e.g., between an electrode pair) is theninsufficient to associate and/or couple any additional nanowires. Hence,by simply correlating the resistance between a pair of electrodes withthe number of nanowires that become coupled, a threshold resistance canbe determined. Then, by proper selection of the threshold resistor R₁, apassive monitoring system can be set up such than when thispre-determined number of nanowires are coupled onto the electrodes, theresistance drops below the threshold resistor R₁, and the e-fielddeposition can not continue. Hence, a passive monitoring system isprovided.

In addition to determining the number of nanowires deposited at anelectrode pair, the monitoring methods of the present invention can alsobe used to determine if undesired versus desired nanowires have beendeposited, and/or if nanowires (whether desired or undesired) havedeposited in a desired or undesired fashion.

For example, individual electrode pairs can be monitored, e.g.,electrically, to determine when a desired nanowire(s) has aligned on theelectrodes. As discussed throughout, nanowires for deposition are oftencore-shell-shell in structure. However, some of these wires may have anincomplete or damaged outer shell or other defect in their structure,and thus be considered “undesired.” The methods of the present inventioncan be used to detect when such undesired nanowires are deposited byobserving a change in an electrical property at the electrodes (e.g.,current or impedance) versus when desired nanowires are deposited.

In addition, as discussed throughout, nanowires can often deposit on theelectrodes in clumps, or as broken, branched or crossing nanowires,rather than straight, single wires, as desired. By monitoring theelectrode pairs via an electrical signal (e.g., impedance, current,etc.), it can be determined if nanowires have deposited on the electrodepair in a desired or undesired manner.

If it is determined that undesired nanowires have been deposited, or ifnanowires have deposited in an undesired manner, the voltage to theelectrode pair can be turned off (or otherwise reduced or modulated)individually, leaving the other electrode pairs electrified, such thatthe undesired nanowire(s) can be removed from the electrode pair, forexample via flushing away, gravity or other removal technique. After theundesired nanowire(s) have been removed, the electrode can bere-electrified and deposition continued. If it is determined thatdesired nanowires have been deposited, the voltage to the electrode paircan be modulated as described throughout so as to lock the desirednanowires onto the electrode pair. Thus, the present invention providesmethods for aligning and depositing desired nanowires via monitoring andcontrol of individual electrode pairs.

In a still further embodiment, the present invention provides substratescomprising one or more electrode pairs, wherein a pre-determined numberof nanowires have been positioned on the electrode pair, and wherein thenumber of nanowires has been controlled according to the methods of thepresent invention. As described herein, the methods, apparatus andsystems of the present invention allow for the control of nanowiredeposition such that a pre-determined number of nanowires can be coupledon to an electrode pair, and then deposition stopped, such that noadditional nanowires are deposited. By monitoring and controllingindividual electrode pairs, it is possible to prepare multiple electrodepairs, each of which comprise a pre-determined number of nanowires.

In suitable embodiments, a plurality of electrode pairs on a substrateare each individually monitored (though more than one electrode pair canalso be monitored together) such that when a pre-determined number ofnanowires is coupled onto the electrode pair, deposition is stopped(suitably deposition is stopped only with regard to that specificelectrode pair) but deposition continues at other electrode pairs. Theend result of the methods of the present invention is that eachelectrode pair comprises substantially the same number of nanowires. Thepresent invention therefore provides for a substrate comprising at leasttwo electrode pairs, suitably at least four electrode pairs (e.g., atleast 5, at least 10, at least 20, at least 30, etc) and at least twonanowires, suitably at least four nanowires (e.g., at least 5, at least10, at least 20, at least 50, at least 100, etc) positioned on eachelectrode pair, wherein each of the electrode pairs comprisesubstantially the same number of nanowires. As used herein the phrase“substantially the same number of nanowires” means that the number ofnanowires positioned on an electrode pair deviates from the number ofnanowires positioned on another electrode pair (undergoing the samedeposition process and control) by less than about 70%. Suitably theelectrodes comprise substantially the same number of nanowires such thatthe number of nanowires deviates less than about 60%, less than about50%, less than about 40%, less than about 30%, less than about 20%, lessthan about 10%, less than about 5% or less than about 1%.

In still further embodiments, the present invention provides systems foraligning and/or positioning nanowires on a substrate, comprising asuspension comprising a plurality of nanowires (e.g., a nanowire ink) asubstrate comprising one or more electrode pairs, and a signal generatorfor generating an alternating current (AC) electric field between theelectrode pairs, and for modulating the AC electric field. Suitablesubstrates and materials for use as electrodes are described throughout.Examples of sources for generating an AC electric field between theelectrode pairs include, but are not limited to, direct electricconnection(s) to the electrode pairs, waveguides and similar apparatus,as well as other equivalent AC electric field sources.

In additional embodiments, the systems of the present invention furthercomprising means for flowing the suspension comprising a plurality ofnanowires over at least one of the electrode pairs. Exemplary flowingmeans include, but are not limited to, a fluid control system forcontrolling fluid flow of the nanowire suspension (e.g., a pump, orsimilar device), a rudimentary device such as a reservoir or otherreceptacle which can be used to pour the suspension over the electrodepairs, and other similar and equivalent devices. Suitably, the flowingmeans is a fixture adapted to be coupled to the underside of thesubstrate. For example, the flowing means is attached, eitherpermanently or removeably, to the underside of the substrate such thatthe entire system or apparatus can be re-used from deposition todeposition. In such embodiments, an apparatus or system of the presentinvention can be utilized to align and deposit nanowires on electrodepairs, the nanowires can then be transferred or removed from theelectrodes, and then the apparatus can be used again in a subsequentdeposition process. As such, the apparatus and systems of the presentinvention can be utilized over and over, generally not requiringreplacement of electrodes or other components until they have worn outor otherwise become ineffective.

In additional embodiments, the systems of the present invention furthercomprise an optical imaging system for visualizing the nanowires, suchas a microscope, an infrared or laser detector, or similar device. Thesystems of the present invention can also further comprise one or morefield electrodes for manipulating the nanowires in suspension on thesubstrate. As discussed throughout, in suitable embodiments the systemsof the present invention further comprises a signal monitoring devicefor determining the signal at the one or more electrode pairs (e.g., aoscilloscope) and means for stopping the AC electric field when thesignal attains a pre-set value. Means for stopping the AC electric fieldwhen the signal attains a pre-set value, include, but are not limitedto, reducing the electric field between the electrode pair, therebystopping the position of the nanowires on the electrode pair (inembodiments where the methods of the present invention are beingutilized to align and deposit nanowires), removing the source ofnanowires (e.g., a nanowire suspension), removing the electrodes fromthe source (e.g, by pulling the substrate out of the suspension) or byother suitable or equivalent methods dependent upon the depositionmethod being utilized.

In further embodiments, the present invention comprises methods fordepositing one or more nanowires on a substrate. One or more nanowiresare first positioned on a substrate, and then the nanowires are heatedso that they become deposited on the substrate. Nanowires can bepositioned on the substrate using any suitable process, for example,electric field alignment, langmuir-film alignment or flow alignment. Inexemplary embodiments, the nanowires are positioned on the substrateusing the various electric field alignment methods as described herein,in which the nanowires are first associated with a pair of electrodes,and then suitably coupled onto the electrode pair by modulating theelectric field, as described herein. For example, FIG. 17A showsnanowires 208 that have been coupled onto a pair of electrodes (204,205) positioned on substrate 202 (e.g., a glass substrate).

Following positioning of the nanowires (e.g., coupling to an electrodepair), nanowires 208 are then heated so as to deposit them on substrate202. As used herein, the terms “heated” or “heating” comprise variousmethods for increasing the temperature of the nanowires (and thereforethe substrate), including, but not limited to, heating in an oven orannealing chamber, heating the substrate itself, for example, via ohmicheating or conductive heating, or other suitable methods.

In general, the nanowires are heated to a temperature of greater thanabout 100° C., for example, about 110° C., about 120° C., about 130° C.,about 140° C., about 150° C., about 160° C., about 170° C., about 180°C., about 190° C., about 200° C., about 210° C., about 220° C., about230° C., about 240° C., about 250° C., or higher. As used herein, theterm “depositing temperature” refers to the temperature to whichnanowires 208 are heated in order to deposit them on substrate 202. Thetemperature of the nanowires can be increased from the temperaturefollowing coupling (e.g., ambient temperature (about 22-25° C.) orabove) to a temperature where the nanowires deposit on the substrate(e.g., greater than about 100° C.), using various heating rates. Forexample, the temperature can be increased to the depositing temperatureover a period of a few minutes to several hours. For example, thetemperature can be increased from the temperature following coupling tothe depositing temperature over a period of about 5 minutes to about 30minutes.

Once the depositing temperature is reached, the nanowires are suitablyheld at this temperature for a period of about a few minutes to severalhours. For example, the nanowires can be held at the depositingtemperature for about 5 minutes to about 2 hours, or about 5 minutes toabout 1 hour, about 5 minutes to about 30 minutes, about 10 minutes toabout 30 minutes, or about 20 minutes. During this period of heating,the nanowires are suitably contacted with one or more gases, includingboth reactive and non-reactive (i.e., inert) forming gases, for example,H₂, N₂, He, Ne, Ar, Kr, Xe, or Rn. In exemplary embodiments, thenanowires are heated in the presence of H₂, for example a mixture of H₂and N₂. While not intending to be bound by theory, the addition of H₂ tothe nanowires during the heating appears to enhance the formation ofbonds/associations between nanowires 208 and the substrate 202, perhapsvia hydrogen bonding between the nanowires and substrate. The presentinvention also encompasses the use of additional gases, suitably H₂comprising forming gases, that allow for the association/boding betweennanowires and substrate. In additional embodiments, covalent bondsbetween the nanowires and substrate may be formed as a result of therelease of H₂O molecules following a chemical reaction.

Following holding the nanowires at the elevated temperature, thenanowires are then cooled to room temperature, for example by removingthe source of heat and providing additional H₂ gas (e.g., H₂/N₂ gas)until the nanowires are cooled.

In exemplary embodiments, prior to heating the nanowires to thedepositing temperature, the nanowires are suitably exposed to one ormore cycles in which a gas provided to the nanowires, and then the gasis removed. For example, N₂ gas can be provided to the nanowires,suitably for about 5-30 minutes (e.g., about 10 minutes) at roomtemperature. The gas is then removed, for example through the use of avacuum pump (e.g., vacuuming to a pressure of less than about 100 mTorrfor about 5 minutes). In suitable embodiments, the gas providing/removalcycle can be repeated, for example, about 2-10 times, and suitably atleast 5 gas providing/removal cycles are utilized prior to heating thenanowires to the depositing temperature.

In additional embodiments, the present invention provides methods fordepositing one or more nanowires 208 on a substrate 202. For example,one or more nanowires are provided in a suspension proximate to anelectrode pair 207 (204/205) on the substrate. The electrode pair isthen energized, whereby the nanowires become associated with theelectrode pair, and the energizing is then modulated, whereby thenanowires become coupled onto the electrode pair (exemplary associationand coupling frequencies and amplitudes are provided throughout). A gasis then provided to the nanowires, and then removed (e.g., via avacuum). This cycle of gas providing and removal can be repeated severaltimes, for example, five or more times. Then, the nanowires are heatedto a temperature of greater than about 100° C. (e.g., about 200° C.),suitably in the presence of H₂ gas (e.g., H₂/N₂), whereby the nanowiresbecome deposited on the substrate.

In embodiments of the present invention in which the nanowires 208 arepositioned using electrodes on the surface of substrate 202 (e.g., viaassociation and coupling as described throughout), electrodes (e.g., 204and 205) are then suitably removed. As shown in FIG. 17B, removal ofelectrodes 204 and 205 leaves nanowires 208 deposited on substrate 202in an aligned/oriented manner. Exemplary methods of removing electrodes204 and 205 include, but are not limited to, physical removal such asscraping or etching, including dry etching, plasma or electron beametching, and chemical etching, such as wet etching, for example anitride acid-based chemical etch. As shown in FIG. 17B, followingremoval of electrodes 204 and 205, aligned, deposited nanowires 208 areleft on substrate 202. Subsequent washing with DI water (or othersolution), as well as drying (e.g., using a spin rinse dryer) do notdisturb nanowire 208 alignment on, or association with, substrate 202.

Additional methods for depositing nanowires 208 on substrate 202 includethe application of an electro-static force to anchor the nanowires, aswell as chemical surface treatment of nanowires 208 and substrate 202 topromote a covalent (e.g., chemical) reaction or non-covalent interactionbetween the two surfaces.

The present invention also provides systems for manipulating nanowiresin a solution (i.e., a suspension of nanowires), for example, system1800 as shown in FIG. 18. In exemplary embodiments, system 1800comprises one or more electrode sets (e.g, 1802, 1808). Each electrodeset comprises a first electrode having a first polarity and a secondelectrode having a second polarity. The first polarity (first electrode)is opposite the second polarity (second electrode) as demonstrated inFIG. 18. As used herein, a “set” of electrodes refers to two electrodes.As shown in FIG. 18, electrode sets 1802 and 1808 can be connected tosingle electrical connections, 1804 and 1806, with electrodes of thefirst polarity connected to one electrical connection (1804) andelectrodes of the second polarity connected to an additional electricalconnection (1806). In other embodiments, each electrode in the varioussets can be connected to a separate electrical connection. As shown inFIG. 18, in exemplary embodiments, the first electrode set 1802 andsecond electrode set 1808 are arranged in an alternating manner (i.e.,no two electrodes having the same polarity are arranged directly next toeach other). Suitably the sets of electrodes will be in the same plane,though in alternate embodiments, they can be in different planes.

A signal generator or other suitable device is then attached to theelectrode sets in order to generate an alternating current (AC) electricfield between the first and second electrodes (having first and secondpolarities). Nanowires that are in solution are polarized as a result ofthe alternating current between electrodes (+, −) of the set andmanipulated in the solution, in response to the generated electrostaticfield. As discussed below, by alternating or pulsing the current at thevarious electrode sets, nanowires can be manipulated in flow channel206. In addition to the use of an AC field between electrodes of anelectrode set, a DC field can also be used to generate adielectrophoretic or electro-osmotic (fluid motion) force between theelectrodes. In embodiments which utilize a DC field, electrodes of anelectrode set are generally separated by greater distances than when anAC field is utilized. This allows for nanowires to be manipulated overthe entire distance between the electrodes, and thus moved along flowchannel 206.

In further embodiments, nanowire manipulation systems 1800 of thepresent invention can further comprise a suspension comprising aplurality of nanowires 208, and a substrate 202 comprising one or moreelectrode pairs 207 (e.g., electrodes 204, 205). The systems suitablyfurther comprise a signal generator or other device for generating analternating current (AC) electric field between the electrode pairs, andfor modulating the AC electric field. In general, substrate 202comprising electrode pairs 207 is oriented opposite the electrode sets(1802, 1808). For example, as shown in FIG. 18, electrode sets 1802 and1808 are oriented above substrate 202, and suitably, are substantiallyparallel to substrate 202. However, the electrode sets can be orientedin any spatial orientation relative to substrate 202 and electrode pairs207, for example, the electrode sets (1802, 1808) can be below theelectrode pairs 207, or can be above the electrode pairs 207, but arenot required to be parallel to substrate 202. For example, electrodesets 1802, 1808 can be oriented at any angle relative to substrate 202.

In suitable embodiments, system 1800 further comprises a means forflowing the suspension comprising a plurality of nanowires over at leastone of the electrode pairs 207 and over at the electrode sets 1802,1808. Exemplary flowing means are described throughout. Additionalcomponents of system 1800, for example optical imaging systems forvisualizing the nanowires, flow control systems and signal monitoringdevices are described throughout.

The present invention also provides methods of manipulating nanowires ina solution, as represented in flowchart 1900 of FIG. 19, with referenceto system 1800 of FIG. 18. In step 1902 of FIG. 19, one or moreelectrode sets 1802, 1808 are provided. Each electrode set suitablycomprises a first electrode having a first polarity and a secondelectrode having a second polarity. As shown in FIG. 18, in suitableembodiments, the electrodes are alternated such that electrodes ofsimilar polarity are not directly next to one another. In step 1904 ofFIG. 19, an electrode set (1802) is energized, wherein nanowires aremanipulated in the direction of the energizing. As used herein, the term“energize” refers to any suitable mechanism or system for providing anelectric current to the electrodes of an electrode set. “Energizing”refers to the generating of a DC electric field and/or an AC electricfield at/between the electrodes of an electrode set.

In step 1906 of flowchart 1900, the energized set of electrodes (1802)is then de-energized. As used herein, the term “de-energized” means thatthe electric current is stopped or otherwise removed from the electrodeset. In step 1908 then, an adjacent electrode set (1808) is energized.As used herein, an “adjacent electrode set” refers to an electrode setthat is immediately next to the set of electrodes (1802) that wasenergized in step 1904, and then de-energized in step 1906. Adjacentsimply refers to the fact that the electrode sets are oriented spatiallynext to one another, but does not require that the electrode sets aretouching or are any particular distance from one another.

As shown in step 1910 of flowchart 1900, steps 1906 and 1908 are thensuitably repeated. For example, energized electrode set 1808 isde-energized, and an additional adjacent set of electrodes (not shown)is energized. Step 1910 can be performed any desired number of times(i.e., energizing and de-energizing adjacent electrode sets). Thiscycling of electrode sets generates a dielectrophoretic force in adirection, suitably the direction of the energizing (i.e., the directionin which adjacent electrode sets are being energized and thende-energized). As described throughout, the dielectrophoretic forcemanipulates the nanowires in the same direction as the applied ACelectric field. Nanowires move from energized electrode sets, toadjacent energized electrode sets, when the previously energized set isde-energized. This cycling of energizing and de-energizing of adjacentnanowire sets allows nanowires to be manipulated in any desireddirection.

For example, as shown in FIG. 18, the energizing/de-energizing generatesa dielectrophoretic force in flow channel 206 in the direction of theblock arrows. Thus, nanowires originally at the position designated bynanowire 208, migrate through flow channel 206 in the direction of thearrows, e.g., along a path similar to that followed by nanowire208→208′→208″, in response to the “wave” or “pulses” of alternating setsof energized and de-energized electrodes (1802→1808→). In furtherembodiments, in addition to the dielectrophoretic force generated by theenergizing and de-energizing described in FIG. 19, nanowires can beflushed away in the same or similar direction, for example, by using afluid flow to aid in nanowire manipulation. Exemplary methods forapplying and regulating fluid flow are described throughout.

In suitable embodiments, the energizing of the electrode sets comprisesgenerating an AC electric field between the electrodes of the sets. Ingeneral, a signal generator or similar apparatus is connected to theelectrode sets, for example, via electrical connections 1804 and 1806.Exemplary AC electric fields suitably comprise a frequency of about 10Hz to about 1 kHz and an amplitude of about 1-10 V (peak-to-peak).

The present invention also provides methods for manipulating one or morenanowires. As discussed throughout, nanowires are suitably associatedand then coupled onto one or more electrode pairs 207 using modulationof an electric field between the electrode pairs. Suitably, uncouplednanowires are removed from the electrode pairs 207 by manipulatingnanowires 208 with sets of electrodes (1802, 1808) comprising a firstelectrode having a first polarity and a second electrode having a secondpolarity, as described herein, and shown in FIG. 18. For example, anelectrode set 1802 is energized, then de-energized and an adjacentelectrode set 1808 energized, thereby generating dielectrophoreticforce, such that uncoupled nanowires 208 are manipulated in thedirection of the energizing, and thereby removed from electrode pair207. For example, as shown in FIG. 18, nanowires 208 are suitablymanipulated in flow channel 206 in the direction of the block arrows.Removed nanowires can then be re-used in subsequent association/couplingapplications by simply recycling the nanowire “ink” or suspension.

Exemplary conditions, including AC field characteristics for nanowireassociation and coupling, as well as nanowire manipulation are describedthroughout. In additional embodiments, the nanowires can be flushedaway, for example by flowing a solution, in addition to manipulationusing the electrostatic forces generated by energizing and de-energizingadjacent electrode sets.

The present invention provides additional methods for positioning one ormore nanowires. As described herein, nanowires are suitably associatedand coupled to an electrode pair 207 using the methods of the presentinvention. For example, a suspension of one or more nanowires areprovided proximate to an electrode pair 207 as shown in FIG. 20.Electrode pair 207 is then energized, whereby nanowires 208 becomeassociated with the electrode pair. The energizing is then modulated,wherein by nanowires become coupled onto the electrode pair. Finally,one or more uncoupled nanowires (uncoupled nanowires include nanowiresthat are not fully bound to electrodes and/or are misaligned orcrossing) are removed from the electrode pair. In suitable embodiments,nanowires 208 are removed using a process comprising energizing aremoval electrode 2002, wherein the uncoupled nanowires 208 aremanipulated in a direction, for example, in the direction of the removalelectrode 2002, and thereby removed from the electrode pair 207. As usedherein, the term “removal electrode” includes one or more electrodespositioned at a distance from electrode pairs, such that upon energizingthe removal electrode, nanowires that are to be removed from theelectrode pairs are manipulated toward or in the direction of theremoval electrode.

For example, as shown in FIG. 20, removal electrode 2002 can bepositioned “above” electrode pair 207 positioned on substrate 202.However, removal electrode 2002 can be positioned at any suitableorientation or distance from electrode pairs 207, and it should beunderstood that the present invention is not limited to simplypositioning removal electrode 2002 above the electrode pairs 207. Insuitable embodiments, removal electrode 2002 comprises a singleelectrode, though multiple removal electrodes can also be used. Removalelectrode 2002 is generally positioned inside of flow channel 206, suchthat it is in fluid communication with the flow channel and thereforenanowires 208 that are in suspension in the solution.

Energizing removal electrode 2002 can comprise generating a DC electricfield, an AC electric field, or both DC and AC electric fields (2004) atthe removal electrode 2002. For example, DC electric fields can have anamplitude of about 0.1 V to about 10 V. Exemplary AC electric fieldssuitably have a frequency of about 100 Hz to about 100 kHz and anamplitude of about 5-150 V. Signal generators or other apparatus/devicefor generation of DC and AC electric fields 2004 are describedthroughout and well known to those of ordinary skill in the art.

Application of a DC electric field to removal electrode 2002 manipulatesnanowires 208 (uncoupled or misaligned nanowires) toward removalelectrode 2002. The generation of a positive electric charge at theremoval electrode 2002 tends to move nanowires 208 from the region ofelectrode pairs 207, up into flow channel 206, for example, as shown inFIG. 21A. Flowing a solution in flow channel 206 then removes nanowires208 from the electrode pair region, allowing them to be collected andre-used in subsequent applications. In additional embodiments, anegative electric charge can be generated at removal electrode 2002, forexample, about −0.1 V to about −10 V. A negative charge at removalelectrode 2002 tends to manipulate nanowires away from the removalelectrode 2002, and hence, toward electrode pairs 207. This can helpenhance the nanowire association/coupling phases as describedthroughout.

Application of an AC electric field to removal electrode 2002, inaddition to manipulating nanowires toward the electrode, also tends toalign nanowires 208 in orientations that are parallel to the AC field,for example as shown in FIGS. 21B and C. Thus, in addition tomanipulating nanowires 208 away from the electrode pairs 207, nanowires208 are also aligned in such a way that they have a greater surface areaexposed to the application of a fluid flow (see FIG. 21C). It has beendetermined that the maximum amount of fluid flow is generated at adistance equal to approximately ½ the “height” of flow channel 206(i.e., half-way between the removal electrode and electrode pairs 207).In exemplary embodiments, flow channel 206 is approximately 500 μm high(distance between removal electrode 2002 and electrode pairs 207 onsubstrate 202), thus maximum flow is attained at a distance of about 250μm above the surface of substrate 202. Application of an AC electricfield, in combination with a fluid flow to flush away nanowires 208,provides a very effective method for removing uncoupled/misalignednanowires, and also allows for the recycling of this nanowire ink infurther applications, thereby limiting material loss and expense.

In a further embodiment, both an AC and a DC electric field can begenerated at removal electrode 2002, for example, as shown in FIG. 21B.Generation of both AC and DC fields help to move nanowires 208 away fromelectrode pairs 207, and also align nanowires parallel to a fluid flow.In exemplary embodiments, removal electrode 2002 can be energized inthree separate steps, for example, an AC electric field can first begenerated at the removal electrode, then a DC electric field and an ACelectric field can be generated, and finally, an AC electric field canbe generated at the removal electrode 2002.

Systems/apparatuses of the present invention which comprise removalelectrode 2002 in addition to one or more electrode pairs 207, aids inlarge area nanowire positioning/deposition. With traditional threeterminal electric field manipulation, IR drop can be an issue. However,the use of a large area removal electrode 2002 limits this concern.

Additional methods for manipulating nanowires away from electrode pairs204/205 can also be used in the practice of the present invention. Forexample, electrode pairs 207 (and suitably substrate 202) can betranslated in various directions to dislodge or manipulate nanowires 208so that they can be removed by electrophoretic or electroosmotic forces,or can be flushed away by a fluid flow. For example, substrate 202, andthus electrode pairs 207 can be translated “downward” thereby causingnanowires 208 to move “up-ward” and into a fluid flow and be removedfrom flow chamber 206. Additional methods for manipulating nanowires sothat they can be removed by flushing away with a fluid flow or byelectrophoretic/electro-osmotic forces include, but are not limited to,ultrasound, as well as sound or other vibration-inducing methods fordislodging uncoupled/misaligned nanowires and then manipulating theminto the fluid flow or electrophoretic/electro-osmotic forces.

In still further embodiments, the present invention provides systems forpositioning nanowires on a substrate, for example as shown in FIG. 23.As discussed throughout, such systems suitably comprise a suspensioncomprising a plurality of nanowires 208. Such systems also comprise asubstrate 202 comprising one or more electrode pairs 207 and one or morenanowire-adhering regions 2302. Suitably, these systems also furthercomprise a signal generator for generating an alternating current (AC)electric field between the electrode pairs, and for modulating the ACelectric field.

As discussed throughout, in one embodiment, uncoupled nanowires can beremoved from the electrodes by flushing away the nanowires using a fluidflow. This fluid flow can effectively remove nanowires that have notpinned and/or locked onto the electrode pairs. In some embodiments it isdesirable to have one or more nanowire-adhering regions 2302 positionedon substrate 202. As used herein, the term “nanowire-adhering” meansthat these regions attract and stick, or otherwise bond nanowires totheir surface. Nanowire-adhering regions can be sections of substrate202, or additional structures positioned on substrate 202, to whichnanowires will adhere.

In further embodiments, it may not be necessary to flush away uncouplednanowires using fluid flow. Rather, nanowire-adhering regions can beutilized to simply remove uncoupled nanowires from the suspension ofnanowires. As discussed herein, this allows for the nanowires to berecovered later and utlized in subsequent coupling reactions.

Suitably, nanowire-adhering regions 2302 are positioned such that theyare separated by a distance of between about 10 mm and about 500 cm,suitably between about 1 cm and about 100 cm, or about 10 cm, about 20cm, about 50 cm, about 70 cm, etc., from each other. Nanowire-adheringregions 2302 can be positioned either parallel or perpendicular toelectrode pairs 207. Nanowire-adhering regions 2302 can be any suitableshape and size, for example, plates, rods, bars, or other geometry towhich nanowires can adhere.

As discussed throughout, in suitable embodiments, nanowires 208 have acore-shell-shell structure in which the outer shell is a conductivematerial, such as a metal. In aqueous or alcohol-based solutions, thesenanowires will generally have a negative surface charge. Thus,nanowire-adhering regions are suitably positively charged. However, itshould be noted that in other environments and/or for other nanowirestructures, negatively charged nanowire-adhering regions can also beused, suitably where the nanowires are positively charged. Any materialthat is the appropriate charge can be utilized as nanowire-adheringregions 2302.

In exemplary embodiments, nanowire-adhering regions 2302 suitablycomprise an oxide, such as Al₂O₃, or a nitride, or other substance thatobtains a positive surface charge in an aqueous or alcohol-basedsolution. In further embodiments, rather than using a substrate that ischarged in a particular environment, a charge can be generated innanowire-adhering regions 2302, for example through a positive ornegative voltage being applied to the regions. Then, the polarity of thevoltage can be switched, allowing nanowires 208 to be released fromnanowire-adhering regions 2302, and recovered and recycled for furtherdepositions. Thus, nanowire-adhering regions 2302 can be material thatis separate from or added onto substrate 202, or can be part ofsubstrate 202 that is then modified, for example via electrification, togenerate a nanowire-adhering region.

In further embodiments, as shown in flowchart 2400 of FIG. 24 withreference to FIG. 23, the present invention provides methods forpositioning one or more nanowires 208. In exemplary embodiments, in step2402, a suspension of one or more nanowires 208 are provided proximateto an electrode pair 207 on substrate 202. In step 2404, electrode pair207 is energized, whereby nanowires 208 become associated with theelectrode pair. In step 2406, the energizing of the electrode pair ismodulated, whereby nanowires 208 become coupled onto the electrode pair207. In step 2408 then, one or more uncoupled nanowires 208 are removedfrom the electrode pair by flushing away the uncoupled nanowires, asdescribed herein. In step 2408, when the uncoupled nanowires are flushedaway from electrode pairs 207, they become attached to nanowire-adheringregions 2302 on substrate 202. As discussed throughout, use ofnanowire-adhering regions 2302 not only limits uncoupled nanowires frominterfering with already pinned/locked nanowires, but also reduces thedistance uncoupled/removed nanowires must travel during the flushingaway procedure.

In addition, as discussed herein, in suitable embodiments it is notnecessary to flush away uncoupled nanowires. Instead, uncouplednanowires can simply attach to nanowire-adhering regions that are“downstream” from the electrode pairs. As used herein, “downstream” isused to indicate that the nanowire-adhering regions are positioned afterthe electrode pairs in the direction of fluid flow (if a fluid flow isused). In such embodiments, as uncoupled nanowires pass beyond theelectrode pairs, they attach to one or more nanowire adhering regionsand can then be recovered and utilized in subsequent coupling reactions.

FIGS. 25A-C shows nanowire alignment and deposition in the presence andabsence of nanowire-adhering regions 2302. FIG. 25A shows the boundarythat forms between nanowire-adhering regions 2302 (e.g., SiO₂) andnon-adhering regions 2502 (e.g., molybdenum), showing the sharp contrastbetween the two material surfaces. FIG. 25B shows nanowire alignment anddeposition in a region that is nanowire-adhering, and FIG. 25C showsnanowire alignment and deposition in a region that is non-adhering.

Use of Nanowires Deposited According to the Present Invention inExemplary Devices and Applications

Numerous electronic devices and systems can incorporate semiconductor orother type devices with thin films of nanowires deposited according themethods of the present invention. Some example applications for thepresent invention are described below or elsewhere herein forillustrative purposes, and are not limiting. The applications describedherein can include aligned or non-aligned thin films of nanowires, andcan include composite or non-composite thin films of nanowires.

Semiconductor devices (or other type devices) can be coupled to signalsof other electronic circuits, and/or can be integrated with otherelectronic circuits. Semiconductor devices can be formed on largesubstrates, which can be subsequently separated or diced into smallersubstrates. Furthermore, on large substrates (i.e., substratessubstantially larger than conventional semiconductor wafers),semiconductor devices formed thereon can be interconnected.

The nanowires deposited by the processes and methods of the presentinvention can also be incorporated in applications requiring a singlesemiconductor device, and in multiple semiconductor devices. Forexample, the nanowires deposited by the processes and methods of thepresent invention are particularly applicable to large area, macroelectronic substrates on which a plurality of semiconductor devices areformed. Such electronic devices can include display driving circuits foractive matrix liquid crystal displays (LCDs), organic LED displays,field emission displays. Other active displays can be formed from ananowire-polymer, quantum dots-polymer composite (the composite canfunction both as the emitter and active driving matrix). The nanowiresdeposited by the processes and methods of the present invention are alsoapplicable to smart libraries, credit cards, large area array sensors,and radio-frequency identification (RFID) tags, including smart cards,smart inventory tags, and the like.

The nanowires deposited by the processes and methods of the presentinvention are also applicable to digital and analog circuitapplications. In particular, the nanowires deposited by the processesand methods of the present invention are useful in applications thatrequire ultra large-scale integration on a large area substrate. Forexample, a thin film of nanowires deposited by the processes and methodsof the present invention can be implemented in logic circuits, memorycircuits, processors, amplifiers, and other digital and analog circuits.

The nanowires deposited by the processes and methods of the presentinvention can be applied to photovoltaic applications. In suchapplications, a clear conducting substrate is used to enhance thephotovoltaic properties of the particular photovoltaic device. Forexample, such a clear conducting substrate can be used as a flexible,large-area replacement for indium tin oxide (ITO) or the like. Asubstrate can be coated with a thin film of nanowires that is formed tohave a large bandgap, i.e., greater than visible light so that it wouldbe non-absorbing, but would be formed to have either the HOMO or LUMObands aligned with the active material of a photovoltaic device thatwould be formed on top of it. Clear conductors can be located on twosides of the absorbing photovoltaic material to carry away current fromthe photovoltaic device. Two different nanowire materials can be chosen,one having the HOMO aligned with that of the photovoltaic material HOMOband, and the other having the LUMO aligned with the LUMO band of thephotovoltaic material. The bandgaps of the two nanowires materials canbe chosen to be much larger than that of the photovoltaic material. Thenanowires, according to this embodiment, can be lightly doped todecrease the resistance of the thin films of nanowires, while permittingthe substrate to remain mostly non-absorbing.

Hence, a wide range of military and consumer goods can incorporate thenanowires deposited by the processes and methods of the presentinvention. For example, such goods can include personal computers,workstations, servers, networking devices, handheld electronic devicessuch as PDAs and palm pilots, telephones (e.g., cellular and standard),radios, televisions, electronic games and game systems, home securitysystems, automobiles, aircraft, boats, other household and commercialappliances, and the like.

It will be readily apparent to one of ordinary skill in the relevantarts that other suitable modifications and adaptations to the methodsand applications described herein can be made without departing from thescope of the invention or any embodiment thereof. Having now describedthe present invention in detail, the same will be more clearlyunderstood by reference to the following examples, which are includedherewith for purposes of illustration only and are not intended to belimiting of the invention.

EXAMPLES Example 1 Positioning of Nanowires

As represented in FIGS. 2 and 14 a, flow channel 206 is initially filledwith solvent (IPA) 1402 and subsequently with nanowire (NW) ink 1404 ofa desired concentration. After introduction of a uniform film of NW inkover the wafer surface 202 the electric field is applied to theelectrode (204, 205, 207) pattern. E-field parameters were about f=500Hz and V=1 Vpp. The NWs subject to these conditions are observed to becaptured/associated from suspension onto the electrodes with their longaxis parallel to the electric field direction. The association can occurwith the NWs pre-aligned in a fluid flow or pulled non-aligned from astationary suspension. The NWs are observed to align adjacent to eachother and exhibit a mobility along the electrode bars. This state of NWcapture is referred to as weak NW “pinning” or association 1406 and isillustrated in FIGS. 5 and 8 a. The mobility can be used to achieve auniform high packing density of NWs per unit electrode width. Anexemplary NW deposition density is on the order of 1 NW per micron(distance is along electrode length). The NW pinning is believed tocaused by an equilibrium between the attractive dielectrophoretic forceand the repulsive electro-osmotic force (see FIG. 5). The next step inthe process (referred to as “NW align” 1408 or alignment modulation)involves increasing the frequency to f=10 kHz with the same ACamplitude. This step aligns the NWs parallel to each other in a lessmobile state (strong pinning) possibly due to the higherdielectrophoretic force acting on the induced dipole moment of the NW.The next step in the process involves increasing the AC signal amplitudeto V=4 Vpp at f=10 kHz. This change in E-field parameters initiates aso-called NW “locking” or coupling 1410 onto the electrodes. The lockedNW state is depicted in FIGS. 11 and 8 b. In this state, the NWs exhibitvery little, if any, mobility along the electrode bars (possibly due tovan der Waals forces active between the NWs and the electrodes). The NWsmatched appropriately to the electrode geometry are locked sufficientlyto be robust with respect to high fluid flow shear forces. However, NWsnot matched to the electrode geometry (e.g., short NWs), curly NWs,crossed NWs or branched NWs are observed to be released from theelectrodes during the “NW release” process step 1412. If the desired NWdeposition density is not achieved (as determined in step 1414), thenthe process steps from “NW pin” through “NW release” are repeated(1406-1412). When the desired NW deposition density is achieved (asdetermined in step 1414), then the E-field is switched off and thechannel is flushed with solvent (IPA) 1420 (suitably on the order of afew hundred microliters to several mL of fluid is used). Finally thechannel is allowed to dry through evaporation of the solvent 1422.Several variations of the electrical parameters have been observed toresult in NW deposition. Specifically, the use of amplitude modulationwas found to yield a high degree of parallel alignment of NWs across theelectrodes. In the “NW pin” step a modulation frequency of 100 Hz with100% modulation index (ratio of amplitude of the field:amplitude of thecarrier) was used. Under these conditions the NWs tend to be capturedprimarily in a parallel orientation up to high NW deposition densities(2 NW per micron). Moreover, crossed NWs are found to be unstable andcan be removed by increasing the solvent flow velocity. Exemplaryresulting NW deposition patterns are shown in FIGS. 9 a-9 b.

Example 2 Nanowire Coupling with CNOS Nanowires

Nanowires comprising a CNOS composition (Si core), with a TaAlNoutermost shell, were prepared utilizing standard growth an harvestingtechniques (see e.g., Gudiksen et al (2000) “Diameter-selectivesynthesis of semiconductor nanowires” J. Am. Chem. Soc. 122, 8801-8802;Cui et al. (2001), “Diameter-controlled synthesis of single-crystalsilicon nanowires” Appl. Phys. Lett. 78, 2214-2216; Gudiksen et al.(2001), and “Synthetic control of the diameter and length of singlecrystal semiconductor nanowires” J. Phys. Chem. B 105, 4062-4064).Nanowires were about 22 microns in length and about 100 nm in diameter.Suspensions of nanowires at both moderate density and low density(10-fold dilution of moderate density suspension) were prepared inisopropanol (IPA).

A 500 micron thick, 4 inch diameter quartz substrate was patterned witha plurality of Moly (Mo) electrodes, each of which was approximately 400Å thick, and about 15 microns or about 30 microns in width. Theelectrodes were positioned such that electrode pairs were separated byabout 20 microns (i.e., just slightly less than the length of thenanowires sought to be aligned and coupled). A flow channel preparedfrom Polydimethylsiloxane (PDMS) was then filled with the nanowiresuspension (by injecting a droplet of the nanowire suspension into thechannel inlet. Capillary driven flow then fills the channel initiallyand then flow begins after a few seconds. The nanowires were typicallypre-aligned with the flow parallel to the eventual electric fielddirection.

An initial AC electric field was generated using a frequency of about 10kHz and an amplitude of about 1V. The peak AC field was about 250 V/cm.This low frequency allowed alignment and association of the nanowiresfrom the suspension onto the electrode pairs. The frequency of theelectric field was then modulated to about 10 kHz-300 kHz to allow thenanowires to couple onto the electrode pairs.

The channel was then flushed with IPA to remove uncoupled nanowires. Thechannel and the coupled nanowires were then dried. During the dryingstep the voltage was increased to above about 4V so as to maintain thenanowires coupled onto the electrodes.

The results of this experiment produced nanowire densities of 1 nanowireper about 4-6 microns (low density solution) and 1 nanowire per about1-3 microns (high density suspension). Some crossing or clumping ofnanowires was observed, but generally the wires were aligned andrelatively evenly spaced. In addition, some misalignment and crossingwas observed to occur during the drying process. It was determined thatthe best nanowire deposition occurred for nanowires of a length thatwere closely matched to the gap between electrode pairs (i.e., nanowiresof about 20 microns in length).

Example 3 Nanowire Coupling on 10×10 Array

Nanowires comprising a CNOS composition (Si core), with a TaAlNoutermost shell, were prepared utilizing standard growth an harvestingtechniques (see e.g., Gudiksen et al (2000) “Diameter-selectivesynthesis of semiconductor nanowires” J. Am. Chem. Soc. 122, 8801-8802;Cui et al. (2001), “Diameter-controlled synthesis of single-crystalsilicon nanowires” Appl. Phys. Lett. 78, 2214-2216; Gudiksen et al.(2001), and “Synthetic control of the diameter and length of singlecrystal semiconductor nanowires” J. Phys. Chem. B 105, 4062-4064).Nanowires were about 22 microns in length and about 100 nm in diameter.Suspensions of nanowires at both moderate density and low density(10-fold dilution of moderate density suspension) were prepared inisopropanol (IPA).

A 500 micron thick, 4 inch diameter quartz substrate was patterned witha 10×10 array of Moly (Mo) electrode pairs (i.e., 100 total electrodepairs, 200 total electrodes), each of which was approximately 400 Åthick, and about 30 microns in width. The electrodes were positionedsuch that electrode pairs were separated by about 20 microns (i.e., justslightly less than the length of the nanowires sought to be aligned andcoupled). A flow channel prepared from Polydimethylsiloxane (PDMS) wasthen filled with the nanowire suspension (by injecting a droplet of thenanowire suspension into the channel inlet). Capillary driven flow thenfills the channel initially and then flow begins after a few seconds.The nanowires were typically pre-aligned with the flow parallel to theeventual electric field direction.

An initial AC electric field was generated using a frequency of about500 Hz and an amplitude of about 1V. The peak AC field was about250V/cm. This low frequency allowed alignment and association of thenanowires from the suspension onto the electrode pairs. The frequency ofthe electric field was then modulated to about 10 kHz and the amplitudemodulated to about 4V to allow the nanowires to couple onto theelectrode pairs. The channel was then flushed with IPA to removeuncoupled nanowires. The channel and the coupled nanowires were thendried

The results of this experiment produced relatively uniform deposition ofnanowires on each of the 10 electrode pairs. For the low densitynanowire suspension, a mean number of 4.75 nanowires were deposited ateach electrode pair, with a standard deviation of distribution of 2.2.For the medium density nanowire suspension, a mean number of 10.8nanowires were deposited at each electrode pair, with a standarddeviation of distribution of 2.5. The spatial distribution across thearray was fairly uniform.

Example 4 Multiple Nanowire Deposition Cycles

The process described above in Examples 1 and 2 was utilized to prepare,associate and couple nanowires to electrode pairs arranged in a 66micrometer array. Following the initial association and coupling phases,a density of about one nanowire per 5 microns was achieved (see FIG. 9a).

The association and coupling phases were then repeated in order toachieve a higher nanowire density. The results are represented in FIG. 9b, and a nanowire density of about one nanowire per 1.8 microns wasachieved.

The use of multiple deposition cycles allows for the generation of ahigher density of nanowires while using a smaller concentration orsmaller volume of nanowire suspension, and also achieving fairly uniformelectrode filling.

Example 5 Nanowire Alignment Using Waveguide-Generated AC Field

Nanowires comprising a CNOS composition (Si core), with a TaAlNoutermost shell, were prepared utilizing standard growth an harvestingtechniques (see e.g., Gudiksen et al (2000) “Diameter-selectivesynthesis of semiconductor nanowires” J. Am. Chem. Soc. 122, 8801-8802;Cui et al. (2001), “Diameter-controlled synthesis of single-crystalsilicon nanowires” Appl. Phys. Lett. 78, 2214-2216; Gudiksen et al.(2001), and “Synthetic control of the diameter and length of singlecrystal semiconductor nanowires” J. Phys. Chem. B 105, 4062-4064).Nanowires were about 20 microns in length and about 100 nm in diameter.Suspensions of nanowires at high, moderate, and low density (10-folddilutions) were prepared in isopropanol (IPA).

A 500 micron thick, 4 inch diameter quartz substrate was patterned withapproximately 25 Cr/Au electrode pairs, each electrode of which wasapproximately 1500 Å thick, and about 50 microns in width, and each ofwhich had a “sawtooth” pattern. The electrodes were positioned such thatelectrode pairs were separated by about 20 microns (i.e., just slightlyless than the length of the nanowires sought to be aligned and coupled).A flow channel prepared from Polydimethylsiloxane (PDMS) (100 micronshigh, 5 mm in width, at an angle of about 1.5° with respect tohorizontal) was then filled with the nanowire suspension (by injecting adroplet of the nanowire suspension into the channel inlet). Capillarydriven flow then fills the channel initially and then flow begins aftera few seconds. The nanowires were typically pre aligned with the flowparallel to the eventual electric field direction.

An AC electric field was generated using a waveguide frequency of about2.45 GHz, with an initial power input of 100 W. The total irradiationtime was approximately 5 minutes, and 4 different antenna lengths wereexamined (0.5 mm, 1 mm, 2 mm and 5 mm).

The results of this experiment demonstrated that the AC electric fieldgenerated using a waveguide could be used to align and then couplenanowires onto electrode pairs. Electrodes outside of the waveguide didnot demonstrate any nanowire deposition. At a low density nanowiresuspension, nanowires were deposited at a density of about six nanowiresper 50 microns of electrode width. The medium density nanowiresuspension deposited approximately 12 nanowires per 50 microns ofelectrode width. The high density nanowire suspension depositedapproximately 20-40 nanowires per 50 microns of electrode width.

Example 6 Nanowire Association and Removal

FIGS. 22A-22F depict a series of micrographs showing nanowireassociation, followed by nanowire removal utilizing the various methodsdescribed throughout. FIG. 22A depicts a series of electrode pairs 207positioned on a substrate. The micrograph is taken from above thesubstrate/electrode pairs. Nanowires have been introduced into the flowchannel, and are visible as blurred objects above the electrode pairs.Also present in the flow channel is a removal electrode above theelectrode pairs, although it is not visible in the micrograph as it isabove the visual plane of the optical imaging system used to visualizethe nanowires. In FIG. 22A, no electric field has been generated betweenthe electrode pairs or at the removal electrode.

In FIG. 22B, an AC electric field having a frequency of about 500 Hz andan amplitude of about 500 mV peak-to-peak has been generated between thevarious electrode pairs on the substrate. In addition, a DC field havingan amplitude of about −2.5 V has been generated at the removalelectrode. This negative polarity DC field helps to manipulate nanowires208 toward the electrode pairs 207 at the bottom of the flow channel.The alternating current at the electrode pairs begins to align andassociate the nanowires with the electrode pairs.

In FIG. 22C, nanowires can be seen to begin to associate between theelectrode pairs (207/208) in a very well-aligned manner. Nanowires canalso be seen to associate with the bus lines that supply current to theelectrode pairs (208′). Both the AC and DC currents are maintained atthe same frequencies and amplitudes as in FIG. 22B.

In FIG. 22D, while the AC current is maintained at 500 mV and 500 Hz,the DC current at the removal electrode is switched to about +800 mV.Nanowires 208 that were associated with the bus lines, as well as thoseweakly associated/misaligned with the electrode pairs are drawn upwardin the flow channel, away from the surface of the substrate and theelectrode pairs. Aligned, associated nanowires however are still presentbetween the various electrode pairs (207/208).

In FIG. 22E, the AC current is maintained at 500 mV and 500 kHz, but theDC current is increased to +2.5 V at the removal electrode. Thiscontinues to draw nanowires upward toward the removal electrode, andalso helps to maintain them near the center of the flow channel.Nanowires are then flushed away by flowing a solution of IPA. ComparingFIG. 22E to 22D, a large number of nanowires have been removed from theflow channel.

In FIG. 22F, the flow of IPA is stopped and the current at the DCelectrode is turned off. The AC current at the electrode pairs ismaintained however, continuing to keep the nanowires aligned andassociated with the electrode pairs (207/208). Very few, if any,un-associated/misaligned nanowires are present in FIG. 22F. In suitableembodiments, the current between the electrode pairs is then modulated,as described herein, in order to couple/lock the nanowires to theelectrode pairs. In further embodiments, the AC field can be modulatedand the nanowires coupled to the electrode pairs prior to the generationof a positive DC current at the removal electrode.

Exemplary embodiments of the present invention have been presented. Theinvention is not limited to these examples. These examples are presentedherein for purposes of illustration, and not limitation. Alternatives(including equivalents, extensions, variations, deviations, etc., ofthose described herein) will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein. Suchalternatives fall within the scope and spirit of the invention.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated by reference.

1. A system for positioning nanowires on a substrate, comprising: (a) asuspension comprising a plurality of nanowires; (b) a substratecomprising one or more electrode pairs; and (c) a signal generator forgenerating an alternating current (AC) electric field between theelectrode pairs, and for modulating the AC electric field.
 2. The systemof claim 1, further comprising means for flowing the suspensioncomprising a plurality of nanowires over at least one of the electrodepairs.
 3. The system of claim 1, further comprising an optical imagingsystem for visualizing the nanowires.
 4. The system of claim 1,comprising a fluid flow control system for controlling fluid flow of thenanowire suspension.
 5. The system of claim 4, wherein the flow controlsystem is a fixture adapted to be coupled to an underside of thesubstrate.
 6. The system of claim 1, further comprising one or morefield electrodes for manipulating the nanowires in suspension on thesubstrate.
 7. The system of claim 1, wherein the nanowires comprise acore and one or more shell layers.
 8. The system of claim 7, wherein thecore comprises a semiconductor and at least one of the shell layerscomprises a metal.
 9. The system of claim 1, further comprising a signalmonitoring device for determining the signal at the one or moreelectrode pairs and means for stopping the AC electric field when thesignal attains a pre-set value.
 10. The system of claim 1, wherein thesubstrate comprises one or more metallic elements positioned betweenelectrodes of the electrode pairs.
 11. The system of claim 1, wherein afirst electrode of an electrode pair comprises a greater nanowirecontact surface area than a second electrode of the electrode pair. 12.A system for manipulating nanowires in a solution, comprising: (a) oneor more electrode sets, each electrode set comprising a first electrodehaving a first polarity and a second electrode having a second polarity;and (b) a signal generator for generating an alternating current (AC)electric field between the first and second electrodes.
 13. The systemof claim 12, further comprising a system for positioning nanowires on asubstrate, comprising: (c) a suspension comprising a plurality ofnanowires; (d) a substrate comprising one or more electrode pairs; and(e) a signal generator for generating an alternating current (AC)electric field between the electrode pairs, and for modulating the ACelectric field, wherein the substrate is positioned opposite theelectrode sets.
 14. The system of claim 13, further comprising means forflowing the suspension comprising a plurality of nanowires over at leastone of the electrode pairs and over the electrode sets.
 15. The systemof claim 13, further comprising an optical imaging system forvisualizing the nanowires.
 16. The system of claim 13, comprising afluid flow control system for controlling fluid flow of the nanowiresuspension.
 17. The system of claim 16, wherein the fluid flow controlsystem is a fixture adapted to be coupled to an underside of thesubstrate.
 18. The system of claim 13, wherein the nanowires comprise acore and one or more shell layers.
 19. The system of claim 18, whereinthe core comprises a semiconductor and at least one of the shell layerscomprises a metal.
 20. The system of claim 13, further comprising asignal monitoring device for determining the signal at the one or moreelectrode pairs and means for stopping the AC electric field when thesignal attains a pre-set value.
 21. The system of claim 13, wherein thesubstrate comprises one or more metallic elements positioned betweenelectrodes of the electrode pairs.
 22. The system of claim 13, wherein afirst electrode of an electrode pair comprises a greater nanowirecontact surface area than a second electrode of the electrode pair. 23.A system for positioning nanowires on a substrate, comprising: (a) asuspension comprising a plurality of nanowires; (b) a substratecomprising one or more electrode pairs and one or more nanowire-adheringregions; and (c) a signal generator for generating an alternatingcurrent (AC) electric field between the electrode pairs, and formodulating the AC electric field.
 24. The system of claim 23, furthercomprising means for flowing the suspension comprising a plurality ofnanowires over at least one of the electrode pairs and the one or morenanowire-adhering regions.
 25. The system of claim 23, furthercomprising an optical imaging system for visualizing the nanowires. 26.The system of claim 23, comprising a fluid flow control system forcontrolling fluid flow of the nanowire suspension.
 27. The system ofclaim 26, wherein the flow control system is a fixture adapted to becoupled to an underside of the substrate.
 28. The system of claim 23,further comprising one or more field electrodes for manipulating thenanowires in suspension on the substrate.
 29. The system of claim 23,wherein the nanowires comprise a core and one or more shell layers. 30.The system of claim 29, wherein the core comprises a semiconductor andat least one of the shell layers comprises a metal.
 31. The system ofclaim 23, further comprising a signal monitoring device for determiningthe signal at the one or more electrode pairs and means for stopping theAC electric field when the signal attains a pre-set value.
 32. Thesystem of claim 23, wherein the substrate comprises one or more metallicelements positioned between electrodes of the electrode pairs.
 33. Thesystem of claim 23, wherein a first electrode of an electrode paircomprises a greater nanowire contact surface area than a secondelectrode of the electrode pair.
 34. The system of claim 23, wherein thenanowire-adhering regions are separated by a distance of between about 1cm and about 100 cm.
 35. The system of claim 23, wherein thenanowire-adhering regions are positively charged.
 36. The system ofclaim 23, wherein the nanowire-adhering regions comprise Al2O3 or anitride.